Sam: Well, I found this. (He hands the little book to Dean) Hidden in their library. It's ancient. Written by a priest who went Dark Side. There's a binding spell in here for trapping a reaper.
Sam: I mean, he must have had some kind of reason for saying it, right? Did he know the demon's plans for me? Am I supposed to go Dark Side or something? What else did he say, Dean?
Sam: You think I'll do it, don't you? You think I'll go Dark Side.
Sam: So we tell him the truth. You say Jesse's destined to go Dark Side—fine. But he hasn't yet. So if we lay it all out for him—what he is, the apocalypse, everything—he might make the right choice.
Bobby: But if we ain't...If there's a snowball of a snowball's chance here...that means we're dealing with a Superman who's gone Dark Side. Which means we've got to be cautious, we got to be smart, and maybe stock up on some Kryptonite.
Castiel: Superman going to the Dark Side. I'm still just Castiel.
Dean: If I do go Dark Side, you got to take me out.
Dean: Yeah. Hey, if Cain found a way to live with it after going Dark Side, then I just got to find a way to keep it in check, so haven't had a drink in a week, eight hours of rack time every night, and . . . now this masterpiece.