I don't think you can kill Yes Man,
Spoiler If you tell him you're going to kill him when you first meet him, he just says you can't, because he just jumps to another robot.
. If you kill everyone, the game just gets pretty boring unless you just like spending all your time going kill-->loot-->sell-->wait for respawns-->kill etc
Well, that's it went for me anyway. There are probably ways to make it interesting.