These games turns me into someone I hate. I am always good, and I'm always a pushover. The reason is that I want to do all the quests, every side quest I find, I want to help, and to do that, you pretty much have to stay on the NPC's good side.
The worst example of this is when I was playing fallout 2, and I got to a guy who wanted me to go out to an enemy filled forest, find his house and get his moonshine. I wanted to scream "Asshole, do you know who I am? I'm the fucking protagonist, I'm on a mission to save the world, I don't have time to be your goddamn waiter!". But instead, I said I'd do it... and I died a little inside.
I've been trying to do better, though. For example, in Oblivion, there was a crazy guy who wanted me to spy on his neighbours, and although it took a solid ten minutes of soulsearching agony, I finally had the power to say "You know what? No, I won't help you, you paranoid psycho."