Thing is, even roleplaying-wise I never could find any reason to be an asshole. In Dragon Age for example, my elf was a racist, angry bastard who just killed everything, and he had a reason to; raised in the slums, his soon-to-be wife taken from him, etc etc. No wonder he was a downright bastard to every human he came across (and cold to the rest).
But why would my Wastelander be such an asshole? I never got that in Fallout 3 or New Vegas (note: haven't finished New Vegas yet though), the evil guys like the Powder Gangers were just evil for, well, evil's sake I suppose. I can't work with that. Why would my character murder Three Dog? Why would my character sabotage the NCR? Etc etc, being evil has little reason. Got that in the first Fallout as well