You monster!
where things are looking grim in the Underground and Sans tells you to go to hell)
I think I'll replay it soon and go for the true pacifist ending. Though I read that the game tracks all your kills even if you reload your save or start a new game, so I hope I didn't screw myself out of anything. According to Gamefaqs the only way to completely corrupt your file to the point where you'd need to reinstall the game is to get the full genocide ending, but I guess I'll find out!