Basically the game takes place in a post-apocalyptic Las Vegas that mysteriously dodged most of the nuclear bombs. Play the game and find out why!
Say you want to roleplay as a sociopath. Well, you can kill anyone in the game if you want. There's only a few exceptions so that the player can't fail the main quests. However, you can do those main quests in a way to maximize suffering for everyone.
Alternately you can play as a savior of the wastes, and make things better for everyone involved.
The player can actually get through the main storyline, and a large number of sidequests without killing anyone.