My vampire character in Skyrim enjoys betraying people. She befriended Eric the Slayer and slowly corrupted his mind, talking him into doing more and more wicked things, eventually getting him kitted out with the Ebony Mail and fully charged Ebony Blade. She eventually made him slaughter his home village, starting with his father.
I know it's not a video game but in D&D I killed a dwarf by dipping my balls into it's eye socket and then broke both the arms of a small child who witnessed the event. I do some pretty bad things in video games, but the worst thing I ever did wasn't even on a game I own. My friend let me play on his fallout 3 for a couple of hours and so naturally, being the good friend I am, I blew up Megaton for him. I then quit the game, made my excuses and left. It was about 3 hours later that I received his text message. He got his revenge though, a few weeks later he sold my Fable: Lost Chapters edition.