I let a desire demon have a little girl(DA:O)
I steal everything that isn't bolted to the floor in any game that'll let me.
Left my cousin to a rapist for money, and I didn't even get to keep the money(DA:O again)
Anything I've ever done in Prototype. I killed more people in humorous ways in that game. The number cut in half, the unfortunate soldiers I've accused of being me, the artillery strikes called in on masses of really anything, civilians, infected, military, whoever. No game has exemplified what it's like to be a superman psychopath. To be honest, at first I tried not to kill civilians, but it's so hard not to especially once you get the extendo tentacle so I just stopped caring.
Then there's the putting mines and grenades in people's pockets in Fallout 3, blowing up Megaton, and enslaving people with the Mesmerizer.
Nothing made me regret the action instantly like giving the Illusive Man the Cerberus base. The red light plus the "MUAHAHAHAHA" look on his face, come on Bioware, there are subtler ways to make us think we made a bad decision.