Sam and Castiel have a conversation later in the season where Sam tries to talk to Cas like ‘This is what *I* did wrong, is there anything you want to share?’ but I think that as Cas is an angel and not a human he feels in a different way and feels like it’s all on himself. But we will get to see Cas forgive himself. And you know Sam has forgiven himself because he knows he was trying to do the right thing. He wasn’t trying to be a bad guy, he thought he had to get powerful enough to kill Lilith and he did. He didn’t know that killing Lilith would start the Apocalypse. But yeah, I think it’s something we maybe can expect for next season, about Cas getting to forgive himself.