Zombies are not dead. Dead will not become a Zombie.
I wasn't aware there was a requirement. Can't really prove that it is impossible for headcrabs to attach to already dead hosts. At least, not to my knowledge.
You know, I just remembered a few mutterings from the scientists in HL. Saying how they warned the administrator and he wouldn't listen, didn't have any regard for the consequences. So, upon further review, Breen ain't a good fella
but he isn't necessarily bad either.