Dean: I need you to promise me something.
Castiel: Of course.
Dean: If I do go dark side, you gotta take me out.
Castiel: What do you mean?
Dean: Knife me, smite me, throw me into the frickin’ sun, whatever, and don’t let Sam get in the way because he’ll try. I can’t go down that road again, man.
Castiel: Is the Mark of Cain still affecting you?
Dean: I can’t be that thing again.