It really seems like Dean feels he made some sort of choice that led to Castiel falling aside once he wasn't needed. But that seems hard to square up with the spoilers that he gets protective of Castiel.
Maybe he was protective, but they butted heads over Benny. Dean wouldn't agree to do this without Benny, and Castiel was never happy about it, and always felt hurt over it. So when Cas got stuck behind in Purgatory, and Dean and Benny both got out, Dean feels like he maybe should have stuck by Castiel, even if it meant they had to find their own way out. It wasn't deliberate, it wasn't something he could've predicted, but it still feels like he betrayed him. Like if he'd done it Cas's way, he and Cas could've found a way out together, and Cas wouldn't be stuck down there now.
Re: Cas spoilers
It really seems like Dean feels he made some sort of choice that led to Castiel falling aside once he wasn't needed. But that seems hard to square up with the spoilers that he gets protective of Castiel.
Maybe he was protective, but they butted heads over Benny. Dean wouldn't agree to do this without Benny, and Castiel was never happy about it, and always felt hurt over it. So when Cas got stuck behind in Purgatory, and Dean and Benny both got out, Dean feels like he maybe should have stuck by Castiel, even if it meant they had to find their own way out. It wasn't deliberate, it wasn't something he could've predicted, but it still feels like he betrayed him. Like if he'd done it Cas's way, he and Cas could've found a way out together, and Cas wouldn't be stuck down there now.
That sort of thing would kind of fit, I think.