Conscientious Objector is a Doom 3 mod that uses the first three levels of the commercial game, stripped bare and repopulated, together with some custom code that stops the player from being able to kill anything.

Sure, you can shoot the zombies all you like, and they will fall down.. it's just that they'll get back up again... On top of this, rather than being told you're great, like most FPS games, you've got an aggressive NPC on your case and in your ear, who'd like nothing better than to see you end up as a zombie's breakfast, and is going to let you know any chance he gets.