I mean, of all time. I'm going with killing Valeria (Speaking of, how did he lose her/the armor?) and or sending Franklin to literal Hell. He also kidnapped Franklin and offered his soul for his mother's, that was pretty bad too, although that MIGHT have been a Doombot (It did happen before Siminson's run.). Didn't he also brainwash Kristoff at one point? Doom hasn't done too many truly horrific things (At least compared to the Joker or Norman Osborn.), but those were pretty dang bad.