Half-Life 2 Theory based off information from Half-Life 1

[Seraphim]

Well, here's my theory from Half-Life 1 and how it leads into Half-Life 2. First, though, the basic plotlines of HL1, Opposing Force and Blue Shift should be considered. Anything important is marked in bold.

HALF-LIFE:
- Scientist (Gordon) performs a test using an unknown crystal and a super-charged spectrophotometer-thingy.
- Experiment goes wrong, something with the crystal causes it to rip open dimensional portals everywhere.
- Scientist fights his way through the compound.
- Apparently, government troops are dispatched to kill everything.
NOTE: This is the first minor implication that the government already knows about these aliens. Although most people might not pick it up, it's slightly odd that the government would have dispatched such heavily armed soldiers to a scientific research lab. Furthermore, this is confirmed in Opposing Force.
- Eventually, Freeman finds a scientist who has a device to take him to Xen.
NOTE: This is the first strong implication that we've already been to Xen. Before this, there is no really emphasized implication that humans have visited the Xen world.
- Freeman fights through Xen, encountering dead (HEV Suit) humans along the way.
- Freeman defeats the Nihilanth, supposedly crippling the alien forces, and allowing the G-Man to take over.

Now, the first thing is obviously that the government wanted Xen experimentation for some military use. Whether it be to harvest the aliens as troopers or to use it as an instant teleporter thing, they wanted it.

One of the most interesting things were what the Nihilanth was saying.

He quite clearly says several things in English.

"YOU ARE MAN HE IS NOT MAN FOR YOU HE WAITS FOR YOU"
"THE TRUTH YOU CAN NEVER KNOW THE TRUTH"
"FREEEEEEEEEEMAN!"

Now, it's pretty easy to figure out that the "not man" waiting for you is the "G-Man". So what's the truth? Well, since you can never know it, then the G-Man must be lying. The Nihilanth obviously knows what's going on, since he knows that the G-Man is running the show and that your name is Freeman.

The Nihilanth could know this information any of four ways:

1) Telepathy - He can read minds. This seems quite unlikely, since if he could read your mind when you were in another dimension, he could certainly read your mind in Xen - and thus, send about a million aliens to go kill you. So I don't find this very likely.

2) Aliens - Another theory is that the aliens report back what they see and hear. Again, not entirely likely - since the Aliens probably wouldn't know that the G-Man is waiting, that he is lying, and that your name is Freeman.

3) Someone Told Him - But who? Probably nobody.

4) Prior History - The Humans have had a relationship of undefined length with Xen aliens. They obviously have met - but we don't know for how long. The only really logical conclusion is that the humans betrayed the Xen aliens. The G-Man might even had been some sort of alien generated to be quasi-human that defected, but there's not a lot of information to prove that. However, this seems like the most obvious choice.

So, we know that the Nihilanth knows a lot, the G-Man knows a lot, the Government knows a lot, but you know next to nothing. You know you're being lied to by the G-Man and the Nihilanth attempted to tell you that.

Now, onto Opposing Force -

OPPOSING FORCE:

- Prelude - In the manual (I believe) it states that you have been doing very odd indoor training exercises for weeks. This makes it extremely obvious that the government had very advance knowledge, and that they even planned the Black Mesa Incident. It would have been a gamble but a very good plan - if the Xen creatures were threatening to rip open a portal to the world, it may be best to create the Black Mesa Incident, and then kill the Nihilanth using Freeman and/or Marines.
- The Marines' planes are shot down over Black Mesa. Adrian awakens and starts kickin' butt.
- You discover neat weapons, ones not in Half-Life: things like the displacer gun, etc. This makes it obvious that there was LOTS of portal research. The Displacer Gun is apparently 100% human tech - perhaps the humans came upon Xen this way?
- Adrian stays in Black Mesa during the time when Freeman is fighting the Nihilanth. When Adrian destroys his foe, the G-Man gives him a speech similar to Freeman. Black Mesa is then nuked.

Now, you'll note that this chapter effectively served to show the extent of the human's knowledge of Xen. It's now been directly confirmed that humans have known about Xen for a very long time. It also backs up and confirms that the G-Man controls a good deal of the US Army with indeterminate but very high authority.

BLUE SHIFT:
Nothing really important here... however, there is one big point.

- Portals REQUIRE crystal, and humans were entering Xen on a constant basis to retrieve the stuff. In fact, it has been going on for so long, they built the current base on the old one! It can then be assumed the displacer gun contains a crystal and uses it as "Ammo". This also suggests that crystals are "used up" with each transport. The rapid harvesting of crystals implies that the humans were trying to use Xen as a transport medium.

HALF-LIFE 1 THEORY:

Based off this information, it appears as if the humans found Xen by some teleportation means (perhaps a pseudo-crystal) as they were trying to develop an effective site-to-site transfer device. When they found out that their medium contained a new race, they may have done some form of diplomacy, or something - probably involving the G-Man. Then, of course, relations broke down as humans went to relentlessly capture crystals. The government had the idea of opening a big portal in order to kill the Nihilanth and thus subjugate Xen. This is precisely what they did, as indicated by their preparations. It worked to an extent - Freeman killed the Nihilanth, and him and Adrian Shepherd are contracted by the G-Man.

So where does it fit in that the G-Man is not human?

Well, he's not human... says the Nihilanth.
He's obviously not a Xen creature.

So what is he? Perhaps, a combination of human/alien? Or human/machine? The Combine.

HALF-LIFE 2

Now, Half-Life 2 starts. Gordon has been in some kind of statis for 10 years... meanwhile, the Combine is raising hell all over. The G-Man sends you out on a mission, presumably to stop the Combine.

It's my theory that he works for some third-party group that's related to the Combine but not it. Perhaps he was the result of some Government experiment - or, maybe not. Either way, he's not human and he's not Xen and he's not Combine. The only logical meaning would be that the G-Man is something related to the Combine. Also note that he hasn't aged since 10 years ago... although the graphics are new, you can quite obviously see that he is near the same age.

So what about the Combine?

It's hard to link the events at Black Mesa to the Combine. The only thing I can come up with that's logical is that the government wanted to create some human/alien/machine hybrid, which was the Combine, which then got out of control.

This would explain the G-Man. The G-Man was the first Combine entity, the Government project to make a super-man and a super-soldier. It must have gone wrong sometime between the Black Mesa incident and City 17 Incident.

--------------------
FINAL CONCLUSIONS
--------------------

The Combine - The Combine were a project to meld human/machine/alien into some sort of super-soldier. They went renegade at some point and started tearing stuff up.

Xen Monsters - They're also tearing things up in the future, probably due to some transport mishap or new Nihilanth, or maybe just government fault.

US Government - Tried to use Xen for a medium through which to transport troops, ended up going to war with Xen monsters. They had two plans -

-- Black Mesa
Kill Nihilanth through Black Mesa.

-- Combine
Create super-soldiers through human/machine technology.

G-Man - The first Combine prototype. Works for the US Government. Pretty much de facto power when it comes to Xen - probably very high up.

Dr. Kleiner - He shows up in HL2. This is guy to look out for - he was responsible for Freeman's transfer, AND for Barney Calhoun's assignment to Black Mesa...

So what do you guys think?

 

[Neo_Kuja]

G-Man - The first Combine prototype.

I somehow think that the G-Man is not ruled by the combine .....

 

[Lanthanide]

Hmm, I think it's a bit far-fetched. Basically we don't know enough about who the Combine are to properly speculate about anything involving them. Nor do we have any idea who the g-man is really working for, although he -was- in charge of Black Mesa (some talk about The Administrator being unhappy during the tram ride and Anomalous Materials).

But here's some extra stuff from HL1 lore that you've missed out, that probably isn't important but might help to perhaps flesh out some theories:

The nihilanth also says "I am the last I am the last" and "not you alone not you alone".

The crystals that you destroy in the nihilanth's chamber are (obviously) the same type of crystal that Freeman did the experiment on.

The Xen Controllers (the monsters that fly around, and presumably control the xen slaves) are mini versions of the Nihilanth. Could have some relevance to the "I am the last" statement.

The xen slaves are slaves (woah!), working in the supposed factory in the Interloper level (I finished HL 7 times, ok?), so the Xen race seems to be pretty advanced technologically. Also we can see in the new E3 video that there is a slave working with Dr Kleiner on something. What does this have to do with anything? Beats me, someone else can come up with an idea, I'm just laying out the facts.

I get the general impression, however, that the Xen aliens were really just trying to defend themselves, and that Freeman has be tricked into doing narsty things to them by the g-man (the g-man could have rescued him at any time, but chose not to, instead forcing him to do his dirty work).

 

[Sprafa]

beautiful speculation, but I would say that the Nihilant is the Combine prototype, as it has been shown that he is bio-mech in some parts

 

[Brian Damage]

Interesting theories here...

I'm pretty sure that Gabe's said that the G-Man's not an alien. He was very cagey on what the guy actually is, though...

 

[Lanthanide]

I've probably missed something (that's what you get for going to planet half-life for all your HL2 information...), but why is everyone so obsessed with the combine being bio-mechanical?

 

[Sprafa]

I've probably missed something (that's what you get for going to planet half-life for all your HL2 information...), but why is everyone so obsessed with the combine being bio-mechanical?

look at the Striders and the Gunships.

They are clearly biomechs. I assume that the entire Combine "thing" is the biomech tech, but many disagree.

 

[NeLi]

The Combine - The Combine were a project to meld human/machine/alien into some sort of super-soldier. They went renegade at some point and started tearing stuff up.

But they're not really super soldiers, right? What's so super about them?

Unless they started reproducing "weaker" versions of themselves after they got free, it doesn't make sense that the government would produce these weak dudes you see in the videos.

 

[Brian Damage]

Well, we see the Soldiers, and we see the Metrocops, and we've heard in the previews about the big armoured dudes... Maybe it's promotion? Maybe they start out as near-humans, and get upgraded the longer they survive?

 

[Lanthanide]

look at the Striders and the Gunships.

They are clearly biomechs. I assume that the entire Combine "thing" is the biomech tech, but many disagree.

Um, how are they clearly biomechs? They just look like machines to me. What exactly is biological about them? I don't see tentacles or borg-like interfaces on them melding flesh to metal.

Sure, the striders are big 3-legged walking stick things, that doesn't mean they're biological at all, that just means they're big 3-legged walking stick things.

The gunships have rotating propeller blades in them (when it flies low over the city and power lines sway up and down) - what exactly makes them organic?

Until you produce some quotes from Valve or interviews which specifically say the combine forces are biomechanic (or have some bio-mechanic parts), I see absolutely no reason to assume this is the case.


Oh, and don't try and say "the combine all have funny masks" - does this mean that when someone puts on a gas mask, they suddenly become half machine half human? I think not. Also, if you look in the latest E3 vid, there's a combine helmet on Dr Kleiner's table. There weren't any wires or anything coming out of it that might have integrated it with the torso of the person it came from.

 

[Brian Damage]

They're biological organisms that have naturally grown parts that are more mechanical in nature. That's what being biomechanical is all about. Look closer. It doesn't necessarily mean that they have artificial parts; that's called being a cyborg. Though by the looks of it, the Combine biomechs may be both.

The Strider: It's got sensory hairs on its legs. It has an obviously organic carapace. It has musculature, both in its hip joints and in its abdomen.

The Gunship: Yeah, its got a propellor. So? The main point of something being biomechanical is that its biological parts grow into forms and functions more usually associated with artificial machines. Look at the weird blisters on its side and underneath it. Look at the strangely shaped fins. Look at, again, the obviously organic carapace.

Look at the way both of them move. So much more like living things than pure machines.

For the record, I don't think that the Combine troopers themselves are biomechs. Too many usable humans running around to bother making something so similar.

And look closer. That thing on the table is not one of the things that either the Metrocops or Soldiers wear. It's the wrong shape, and it's too rigid-looking. The troopers wear flexible gas masks. Just ask... FlyingDebris, I think it was.

 

[Sprafa]

Besides, the mask could be covering the biomech face, not to cause panic. It doesn't necessarly has to be part of some kind of exo-armor....

 

[Esquire]

Lanthanide: "...you look in the latest E3 vid, there's a combine helmet on Dr Kleiner's table.."

yeah man that's what i though too! But i examined over and over, and i think that it was just a microscope. I think this cos it looked to have 3 black/glassy circles on it, and it looked spheric too. Alot of microscopes have 3viewers on them, you can rotate the globe and the viewer will move round so that you get different magnifications. So yeah, it's a microsope.

Also, i think it was hooked up to one of the computers. Look at the monitor closest to the combinemask/microscope.

 

[DizzyOne]

"YOU ARE MAN HE IS NOT MAN FOR YOU HE WAITS FOR YOU"
"THE TRUTH YOU CAN NEVER KNOW THE TRUTH"
"FREEEEEEEEEEMAN!"

maybe G-man is an alien Iznogood -> "I WANT TO BE THE CALIPH INSTEAD OF THE CALIPH!"

 

[Seraphim]

But they're not really super soldiers, right? What's so super about them?

They would presumably be more durable.

Now, I do admit that some of it is sketchy, especially with the Combine. I personally think the Combine is bio-mech stuff, just because they don't seem to be 100% alien and 100% human. The Strider seems biomech, and the Strider is part of the Combine, so we can infer that the Combine soldiers are some sort of combination of mechanical and human parts.

As for the G-Man, I'm not saying he was ruled by the Combine, just that he was built like the Combine (the first one, perhaps?) That would explain one very important point:

The G-Man somehow managed to traverse all throughout the Black Mesa complex without dying. He has to have some trick - whether it be the pistol in his briefcase or some kind of insta-transport mechanism, he needs to be ultra-strong to be able to go through the base like that without even a HEV suit or some comparable protective device. Thus, you can infer that he has some trump card.

Of course, if you think the Combine is biomech, the G-Man would be built much much better. More money would go into the G-Man than a common foot soldier, which would explain the G-Man's near invincibility. I can imagine that the G-Man can take a clip from an MP5 or a shot from one of those Alien Guards (the ones that fire electricity) and he won't be scathed. He's sort of the pinnacle of the biomech technology.

Perhaps you need to face another like him at the end of HL-2.

EDIT: Also note how human-ish he looks. He seems human in appearance, but his voice is a bit off. He must be a very good bio-mech, then. The troopers would then be of lesser quality - probably so low quality that they need masks to hide the fact that they are biomech.

 

[Brian Damage]

Maybe G-Man's a rogue Combine?

 

[Shuzer]

Interesting theories here...

I'm pretty sure that Gabe's said that the G-Man's not an alien. He was very cagey on what the guy actually is, though...

Gabe already confirmed the G-Man is a hermaphrodite!
I know he was joking, as am I

 

[Seraphim]

Well, since the G-Man precedes the Combine (or at least, the apex of Combine development) by ten years, it's safe to assume that not much of a Combine existed at the time of the G-Man's creation - and even if there was some substantial Combine force, they were still under the dominion of the US Government. Since it was clear that in Half-Life 1, the G-Man was part of the US Government, then Gordon must then have been in stasis under Government protection. Therefore, the G-Man must still be associated with the government - or, at least, NOT associated with the Combine. I think he was just a really expensive "original" Combine that the Government used to work with the Xen aliens. The other Combine soldiers would be much cheaper, but much more plentiful, combat-oriented models.

 

[jabberwock95]

The G-man isn't invincible. The only time you ever would be able to fire weapons at him is if you cheated, and thats not solid info. Good speculation on the combine/gman bit though!

I think the resonance cascade was unplanned, or maybe foreseen but not prepared for. If the military were planning a full-scale invasion, why fill the place with cowardly scientists and rent-a-cops that have no idea whats going on, then kill them all. Too many rogue elements.

Not to mention gordon. They may have expected him to be killed, but surely they would want him to be 'one of theirs' if they were planning a resonance cascade. Plus, the scientists seem pretty brainy, and keeping some form of sabotage from them would be difficult. Why not just staff the place with loyal, informed scientists and heavily-armed military?

 

[jabberwock95]

Well, since the G-Man precedes the Combine (or at least, the apex of Combine development) by ten years, it's safe to assume that not much of a Combine existed at the time of the G-Man's creation - and even if there was some substantial Combine force, they were still under the dominion of the US Government. Since it was clear that in Half-Life 1, the G-Man was part of the US Government, then Gordon must then have been in stasis under Government protection. Therefore, the G-Man must still be associated with the government - or, at least, NOT associated with the Combine. I think he was just a really expensive "original" Combine that the Government used to work with the Xen aliens. The other Combine soldiers would be much cheaper, but much more plentiful, combat-oriented models.

So who's mass-creating the combine? The government surely wouldn't want their planet destroyed...

 

[Eejit]

The G-man isn't invincible. The only time you ever would be able to fire weapons at him is if you cheated, and thats not solid info. Good speculation on the combine/gman bit though!

Eh, wrong. You can shoot at him in a couple of places without cheating. For example, in OpFor when he closes the door stopping you from getting in the helicopter you can shoot at his legs while crouching. You can also shoot at him in Blue Shift where he goes past in a tram on the 2nd level or so.

 

[Lordblackadder]

Look one thing that is well established long before Lambda Core is that the BM facility is meant for:

-Limited teleportation to Xen.
-Collection of specimens from Xen.
At least one scientist even says so.

This is why we encounter all those Xenian specimens in OpFor and Questionable ethics. The HL booklet confirms this.

The army was ready to move into Xen, that is why there are tanks and troops on Xen at the end.

The grunts going into BM to kill everything may have only been told "Freeman started this, kill everyone or there will be an invasion".

So I think that the whole thing was largely done on purpose.

 

[Sprafa]

I'm making the "theory to end with all theories"

I'm so sick of this!

 

[jabberwock95]

Eh, wrong. You can shoot at him in a couple of places without cheating. For example, in OpFor when he closes the door stopping you from getting in the helicopter you can shoot at his legs while crouching. You can also shoot at him in Blue Shift where he goes past in a tram on the 2nd level or so.

You can also shoot him in one place in half life, but you never see him for long enough to say he's invincible, you can only get a couple of shots at him. I'll admit he is really unkillable, im just saying I don't think he's meant to be in the story. I think he's just human.

 

[jabberwock95]

Look one thing that is well established long before Lambda Core is that the BM facility is meant for:

-Limited teleportation to Xen.
-Collection of specimens from Xen.
At least one scientist even says so.

This is why we encounter all those Xenian specimens in OpFor and Questionable ethics. The HL booklet confirms this.

The army was ready to move into Xen, that is why there are tanks and troops on Xen at the end.

The grunts going into BM to kill everything may have only been told "Freeman started this, kill everyone or there will be an invasion".

So I think that the whole thing was largely done on purpose.

Yes, black mesa was designed for teleportation to Xen, but hardly anyone in the facility knows this (confirmed in blue shift, I think). Only high-level scientists knew the real purpose of black mesa, so how come someone ignorant like gordon was stuck in the test chamber if there was meant to be a resonance cascade? If theres one thing the gman hates, it's 'loose ends'.

As for the tanks and marines, thats an issue thats been confusing me too. So far, we haven't seen anything in BMRF capable of taking a tank to the border world, and we're led to believe black mesa is on the cutting edge. So how did they get there? If humanity already had teleporters capable of taking large numbers of tanks and soldiers, why did they need to make a resonance cascade in the first place?

As for them being ready to move in on Xen, thats still a matter of speculation. Remember, half life took place over three days, plenty of time for the government to arrange an attacking force, especially if Xen is as small as the portions freeman visited.

 

[Andy]

Fact: G-man and the Combine are not on the same side. G-man knows Freeman. He didn't set Freeman loose in that kind of situation amid the Combine for their benifit.

 

[Seraphim]

You can also shoot him in one place in half life, but you never see him for long enough to say he's invincible, you can only get a couple of shots at him. I'll admit he is really unkillable, im just saying I don't think he's meant to be in the story. I think he's just human.

Well, it's likely to assume he's unkillable. You have a guy in a business suit carrying a briefcase, unarmed (for all we know - the biggest he could be carrying is a 9mm pistol), walking through an alien-infested, radioactive-ooze-filled, crumbling-with-explosions research facility. He'd HAVE to be unkillable... I mean, how many times did you die in Half-Life? A lot. And you had the Gauss gun. Dude... the Gauss Gun! Imagine playing Half-Life walking all the time, no HEV Suit (no armor) and no guns. How far would you get if you were mortal?

 

[CrazyHarij]

G-Man is running for president in 2010.

VOTE G-MAN!

 

[Seraphim]

I'd vote for him.

 

[Dr. Freeman]

G-Man is running for president in 2010.

VOTE G-MAN!

/me goes votes for G-man :E

 

[RakuraiTenjin]

Well, before he does that I'd like to know some of his stances on immigration and welfare reform.

 

[Neo_Kuja]

You can also shoot him in one place in half life, but you never see him for long enough to say he's invincible, you can only get a couple of shots at him. I'll admit he is really unkillable, im just saying I don't think he's meant to be in the story. I think he's just human.

Go to the beginning part of "Power Up", where you can see Garg fighting the Marines. Look to the control room and the G-Man stares at you. If you're quick enough, you can get about 10 seconds + of shooting time with the 9mm.

In the section after the epiliptic (?) zombie, when you round the corner after shooting the first two houndeyes, you can see him on the railing, and have about 5 seconds + to shoot him.

I think it's impossible to kill him. (until the end of HL3, at least)

 

[Seraphim]

Precisely. Not only can you not kill him, but I'm sure not an Alien or any other of the hazards in Black Mesa can. Providing that he is a biomech, it's reasonable to assume tons of money was funneled into him. In contrast, the Combine soldiers were probably made cheaply. For a comparison, if you're a fan of the Terminator movies, it's sort of like the T-X (Female) and the T-800 (Schwartzenegger). More effort was put into making the T-X, since there would only be a few. The T-800s were stock battle models, meant to just be grunt troopers. The T-800 didn't have anything particularly special, but the T-X was stocked with all kinds of internal weaponry, nanomachines, all kinds of special armor, etc... which is what happens when one model gets a ton of funding and the others are made cheaply.

Furthermore, an advantage to a Combine soldier is their controllability (well, look how that turned out for the US Government!) They would never ask questions, etc. I assume they broke away from the US Government, though. We'll have to see in HL2.

 

[jabberwock95]

Well, it's likely to assume he's unkillable. You have a guy in a business suit carrying a briefcase, unarmed (for all we know - the biggest he could be carrying is a 9mm pistol), walking through an alien-infested, radioactive-ooze-filled, crumbling-with-explosions research facility. He'd HAVE to be unkillable... I mean, how many times did you die in Half-Life? A lot. And you had the Gauss gun. Dude... the Gauss Gun! Imagine playing Half-Life walking all the time, no HEV Suit (no armor) and no guns. How far would you get if you were mortal?

In the lamda reactor section, you can see the gman walking into a portal. Also, in other areas you can get to where he was too quickly for him to have walked off, but he's dissappeared. If the gman has been controlling (or taking advantage of) the portal system in black mesa, he could have travelled all over the facility without putting himself in danger.

 

[jabberwock95]

Go to the beginning part of "Power Up", where you can see Garg fighting the Marines. Look to the control room and the G-Man stares at you. If you're quick enough, you can get about 10 seconds + of shooting time with the 9mm.

In the section after the epiliptic (?) zombie, when you round the corner after shooting the first two houndeyes, you can see him on the railing, and have about 5 seconds + to shoot him.

I think it's impossible to kill him. (until the end of HL3, at least)

Yes, in the game he is invincible. Bullets just bounce off him, he's impossible to kill. But I don't think it's meant to be like that.

Valve wants you to see the gman following your adventures around black mesa, but they don't want you to kill him (otherwise you can't meet him at the end). Obviously they can't always put him behind bulletproof glass or something, so the only option is to make his character unkillable, even if it's not part of the storyline.

Plus, when you see him on the railing, he stays there UNTIL you shoot at him. If you don't shoot him, he'll stay and watch you. If you shoot him, he'll immediately walk away, so you can't shoot him. Why would he do that if he's invulnerable?

 

[ferd]

nice summary
everything he says is true...except when it ends with a question mark

 

[NeLi]

Yeah, it probably was just a design choice made by Valve. I really don't think they had any secret plot plans to make the G-man invincible in HL1.

 

[Eejit]

Unless he's a killer robot from teh futoor with teleport technology in his bodeh!!!!11
[/NOOB]

 

[Seraphim]

It's not far-fetched that he would have some sort of personal teleportation device. He must have a more advanced version than anything that Black Mesa had. Remember at the end of OpFor, he says Black Mesa was the biggest failure. This implies there were other facilities working on similar things.

As for the personal teleportation device, we've seen one before - the Displacer Gun from OpFor. However, that seems a little big for the G-Man to be carrying. Also, the Displacer Gun is rather inaccurate in terms of where it transports. Thus, the G-Man must have an infinitely more efficient and precise version of this weapon. If he is a biomech, it could be concealed in his body - and he might have some sort of ultra-small reactor. It would make sense.

 

[Moejoe]

I'll take a stab as what the deal with the aliens are. I think it's kind of like Point of View. Basically, the aliens were brainwashed and forced into working for Nihilanth, at least the smarter ones were (I.E. the slaves). But in Point of View, you kind of have an epiphany and 'see the light.' You release yourself from slavery and see the way of the humans. The one working in Dr. Kleiners lab is probably like the one i'm talking about. He isn't forced into working for the humans, but he chooses to do so because he likes what the humans are doing. He is a refugee of some sort. The dumber ones, like ant lions, they just attack whatever they see and rely on natural instinct.

As for the G-Man. I don't think he was invulnerable. I just think that killing him wouldn't have made sense. I remember in an interview about HL2 awhile ago, Newell or some other official said something about, "you don't want to kill the g-man." pretty much meaning if you kill the g-man, game over dude. F7, reload last save. in other words, he's not immortal. they just didn't let you kill him in HL1.

I think the theory of him being some sort of 'combine prototype' holds itself up to an extent. He does seem a bit inhuman, at least his voice does. why does he have such long pauses in talking?