Dean has misunderstood and Castiel immediately shakes his head, reaches out and places a hand on Dean's shoulder. "Dean, no. It's not what you're thinking. Your brother is not a demon. He is ... I don't think there's ever been anything like what Sam has become before. He is something new, much like myself. After he died, Crowley attempted to claim Sam's soul for hell, but found that the trials had turned Sam into something else entirely. Sam locked Crowley in the cage with Lucifer and Michael, and released Adam. He's been managing hell for lack of a better explanation Hell itself is still necessary, and he has closed it's gates for the most part - no demon is able to leave, though reapers still have access to ferry damned souls there. Sam can also leave any time he wishes and does so frequently. He's safe, he's fine, he is doing a necessary job and the world is safer and better for it. Do you understand?"