Castiel listened to Sam's prayer for him. Maybe he was right. Maybe he did have to put all those leviathans back. Maybe then he could be redeemed. It had worked for Sam. Sam had let out Lucifer. When he had put him back, all had been forgiven. He had had to suffer greatly, but that was only right. He had to suffer to. If he was sucked into Purgatory forever than so be it. It was what he deserved and he was going to step up and do what was right. Sam was the only one who could have gotten through to him.