User blog comment:Twilight Despair 5/A (Cure) or whatever for the Mark of Cain/@comment-71.191.75.32-20150225212357/@comment-97.84.77.235-20150226052710

Cain said that Dean is living his life in reverse. Cain got the mark when he killed Abel, which began his story, so maybe Dean will have to kill Sam, as killing his brother was the source of the mark.