Sometimes, i start these games thinking "I'm gonna be an asshole" if its an option and then the first choice comes up and i can't do it lol. I am the same way with like Mass Effect, Paragon for life.
I think the problem for me is who you're reacting to. If the NPCs are well-written, likeable, and generally nice people, it makes me feel worse to be an asshole to them. So in stuff like Mass Effect, KotOR, Fallout, etc I tend to wind up being fiercely overprotective of my allies and generally noble when dealing with innocent people. I wind up being a better person in those games than I am in real life.
But in games where you're dealing with terrible people who are kind of assholes themselves, I'm much more inclined to be an asshole back to them. Pay evil unto evil, be a monster to monster. That's where the power fantasy/revenge fantasy appeal comes in. Delivering instant karma to people who absolutely deserve it. Even in cases where they aren't necessarily totally
bad, just doing really annoying things or provoking me (I don't think I've ever
not Renegade interrupted Han'Gerrel in ME3 - *stomach punch* get off my ship!).
Like even in
this game, I was fully prepared going in to deliver righteous wrath upon Rachel and Katherine because of how they were kind of introduced to us, but then you learn more and interact with them and now I'm probably feeling more overprotective of them than I am of Ellie (hence me being pissed at Jada for taking advantage of Rachel).
I've also noticed I have a much harder time being evil when I play as female characters than I do playing as male characters. So much so that my first couple playthroughs in games like KotOR, Mass Effect, Dragon Age, etc are always female characters, but then when I want to do an "evil" run to see alternate endings/get achievements, I switch to a male character instead. It's not even that I feel like women are innocent or pure (women can be incredibly evil in and of themselves), but apparently my own inner female self is just a sweet loving person who feels bad when she makes people feel sad.
But my inner female self is apparently also an incredibly vindictive bitch when you're an unrepentant dick who is out to hurt the people she cares about. Because I will utterly destroy people who come after me and mine. Which is why a certain irritating cop had better be really careful about where he pokes his nose.