I went on one in Fallout 3, but that was really out of boredom then anything else. I decided I wanted to start over, but before I did, I'd attempt to kill every NPC in the game.
I ended up killing:
-Everyone in Oasis, including Harold/Bob.
-Everyone in Rivet City(excluding a certain doctor who kept returning from the grave for unknown reasons)
-Everyone in Megaton(naturally)
-Everyone in Tenpenny Tower. Twice. (Once with the feral ghoul massacre, and once with good ol' fashioned running and gunning)
-Everyone in the Temple of the Union
-Everyone in Paradise Falls(that includes all slavers AND slaves. I'm an equal oppertunity murderer.)
-Everyone in the Canterburry Commons.
-Everyone in the Republic of Dave
-Everyone in Bigtown
-Several "independent" NPCs, including but not limited to:
--The guy that gives you money in exchange for the ears of good NPCs
--That sweet old lady that wants you to find her violin from a vault.
--The crazy Russian Guy with the prostitutes. (and the prostitutes)
--The friendly Super Mutant who's name I forget (Not Fawkes, there's another one. He's just wandering around the wastes, I don't think you can get him to follow you)
I did NOT, however, attack the Citadel. I figured they would eat me alive.