Cas' insistence on staying in purgatory as a form of self-flagullation spoke volumes about his mental state that season, and it is a shame that it was never resolved. Heaven did something fucked up to him as payback, and I guess that made things even in his mind? After Naomi's mind control he only seemed focused on being accepted back into angel society, not so much on redemption for killing hundreds of angels.
Honestly, I could never really care about Cas' standing with the other angels, and I wasn't sure why it mattered so much to him. In S5 he seemed perfectly fine with becoming a pariah, and he didn't bat an eye when he turned his blade on other angels. In S6 he made it clear that he was fighting Raphael to prevent the apocalypse from starting up again, and he seemed mostly frustrated and uncomfortable leading the other angels. It also doesn't help that, aside from Cas, every angel without exception has been a massive dick. Even the fun ones like Gabriel and Balthazar don't really give a shit about human life and mainly looked out for their own interests. And then there was Hannah, maybe the most boring character in the history of SPN (definitely most uninspired angel name) who I guess was supposed to endear us to the angels even after she pulled that whole "kill your best friend right now because I say so or else I'm kicking you out of the club I begged you to make" stunt and straight up ganked a guy who's only crime was wanting to chill out and get his fly fishing on. At least guilt made sense as a motivating factor, but if anything, Cas' desperation for acceptance in his toxic "family" runs contrary to the show's themes.
I guess what I'm saying is that revolving Cas' entire character around seeking the approval of these winged dipshits for at least 3 seasons killed him. He built up so much pathos by S8, and then they just dropped it to keep him in a stagnant loop as a plot device.
Balthazar was pretty fun, though. He deserved more episodes.