[Brandoom]

Zombies are always so mindless. Perhaps the genius scientist is only partially infected because he has taken the antidote but it was too late for it to work properly. Now he is an intelligent zombie and is leading the commandos to the stockpile of antidote to destroy it, all the while telling them it is the zombie pathogen. As they proceed his true zombie like nature becomes more and more apparent because he is smelly, eyeballs falling out, and other fun stuff. This is really great because you can have the context soooo dark that no one can see outside of their torchlight, just hear. something like "plop! you hear a strange noise, a bit like a chunk of jell-o hitting the floor, but you failed your save and stepped on it, and it makes a popping sound as the most putrid stench you have ever smelt now emanates from your boot." Now the player has the zombie madness and no one else in the party knows it, but they might be able to smell it.

But maybe the scientist convinces the party to destroy the only hope left for escaping the zombie plague (the antidote), and after that heartbreaking realization the game becomes much more dark. this would introduce the nightmare part of the story where Billy the explosives specialist realizes he is becoming a zombie, and that there is no escape, and the merciful thang to do, by god, is to blow the &*$% out of everything.

All the while, the scientist has made his escape. He decides that Hitler is a pansy and he can do a much better job. He leaves the commandos for dead, not realizing that they would find only X vials of antidote left unbroken, and are now some pissed off zombie killing machines. Deep within the Nazi fortress and immune to the plague, the dumb zombies are no match for their weaponry. Especially once they uncover the stockpile of sonic/laser/missle/robot/psionic/whatever the hell you think is cool weapons that allow them to lay waste to all but the most heinous of the hybrid scientist's super zombie creations. But they have to deal with insane humans now too.

Meanwhile, after he ate Hitler's brains and has sewn Hermann Goring's lovely blond scalp to the necrotic flesh clinging to his skull, the scientist becomes a narcissistic crazy pants who is looking for the perfect abs/nose/whatever is handy body part to stitch onto his lovely zombie body. Think that episode of Invader Zim where Zim says" More organs means more human"

I think you could have alot of fun with this story line but you might want to keep it a little more serious than I have here. If that is the case I would focus on terror. limited sight distance, isolation, constant threat of zombie brain munching, and really creepy sounds.