Supernatural season finale theory
Okay so what if in the season 10 finale ..Dean does kill Sam. But when he does kill Sam , the mark start to vanish . It makes sense because of what Cain said ..“ you are living my life in reverse ”
Cain killed Abel and then got the Mark of Cain.
Dean already has the Mark of Cain and kills Sam and the mark vanishes.
…but then what the hell would next season be about?!