The most despicable, awful and down right evil thing I’ve ever done in a video game was during a Dark Side playthrough of Star Wars: Knights Of The Old Republic. At the end of the side quest Honest Debt, you convince a man to either spare or gun down a rather horrible chap who wronged him greatly. There’s nothing particularly remarkable about that - I’ve encountered the same thing dozens of times in games - but what comes after is a real doozy.
Not content simply with nudging the fella into giving in to his worst impulses, I then proceeded to persuade him to wipe the very memory of his nemesis from the galaxy, up to and including hunting down his friends and family members. Bastilla Shan, noted Jedi do-gooder, pointed out that neither she nor the Jedi council would approve. I was giggling like a schoolgirl.