In 2147, two years after the Mars Base fiasco, the Union Aerospace Corporation drove the Expendable Resource Reconnaissance Bill through World Government. Primarily, the bill was intended to avoid further waste of valuable resources when returning to the sealed off facility. The average space marine cost over two million dollars to train and equip - each lost marine represented a serious drain on UAC resources. The ERRB solved this problem by ensuring a steady supply of low cost resources for missions with high casualty rates from maximum security prisons. The most valuable assets came from the Glasshouse: marines who had refused to bear arms. These prisoners were highly trained, psychologically stable and displayed strong survival instincts. And, of course, if they never made it back, they wouldn't be missed. You're going back to Mars. But this time, your shotgun has only got rubber bullets in it. Conscientious Objector: killing is for wimps.
This mod by chineseroom overhauls the game and creates a new storyline. You fight with rubber bullets, and have to try to end the campaign without killing any monsters.
Conscientious Objector is a Doom 3 mod created by thechineseroom, a research project building experimental FPS games based at the University of Portsmouth, UK.
It 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.
More information about the mod can be found on the project website. Look out for the other two mods being released this summer: Dear Esther (Half Life 2) and Antlion Soccer (Half Life 2 Deathmatch)