In Bioshock when... well, when the twist happens. So I don't spoil it for anyone who hasn't played the game (You damn well should), all I'll say is, that I felt like it was me, not the guy I was playing as, who had been manipulated.
Also, in Black Ops when <spoiler= SPOILAARS> Dragovich, Steiner, and Kravchenko kill Dimitri (You) for a worthless reason. I'm with Reznov, he should've died in Berlin like a hero.
And when that Russian POW Camp commander forces you to play Russian Roulette with Bowman, and when you and him refuse, he smashes Bowman's head in. Most satisfying enemy kill in all of Call of Duty is when you chase him down, and as the fucking coward tries to open a hatch door, I emtpied the whole damn AK mag into him.
The fucking prick. [/spoiler]