Fallout, a game set in an alternate universe where technology didn't seem to advance much past the... 50's i would say. except they have intelligent A.I. and junk like that. Anyways, the games take place after a nuclear war, in which nukes were launched, turning the world into a wasteland where mutants, zombies, giant scorpions, etc. roam the land. You are a survivor, usually at first living in essentially an underground bomb shelter, with other people who've been there for years. You just wander around the wasteland pretty much doing whatever the hell you want. You don't have to kill anyone, but sometimes the story requires it, or you get stuck in a bad situation, or you just do want to kill everyone.