
This got long, so buckle in.
Consider Cas' story, Anon.
This guy defied literally everything he is and knew in order to learn free will from the WInchesters. He developed - or at least truly felt - emotions for the first time because of them, and because of Dean. He was demoted (On The Head of a Pin), then captured and tortured into compliance by Heaven (The Rapture), then hunted by Heaven for rebelling (S5), then forced to kill his own brothers and sisters to keep Dean and Sam safe (S5, S6). He's died, over and over, for them.
99% of what he did in the S6 finale that is being treated as so unforgivable was done to protect Dean and Sam. (1% being Sam's wall, which I'll come to in a minute.) His crime wasn't working with demons - all of them have done that before - it was lying to them and doing it behind their back. I'm not pretending he didn't do some terribly shady things (torturing Ellie from Purgatory springs to mind) but again, this is the Winchesters. They do not get to throw stones here, their houses are made of sugar glass when it comes to 'shady things in the name of good'. (How high a body count do they have just from killing demons in their hosts, again? Sam draining that nurse before killing Lilith? Dean torturing demons in the same episode as the Ellie thing?) But anyway, y'know, I get being angry about all that. Especially the lying. I do! Especially when someone has so many issues with people consorting with demons secretly as Dean does. Especially when Dean defended him only to be proved wrong. I get taking it badly, honestly I do. But not that badly, surely? Not after everything Cas has done to prove himself a loyal friend with their best interests at heart? Surely he's earned a little understanding, a little benefit of the doubt? Nope! Apparently not! He's gonna get yelled at, accused of terrible things like bringing Sam back soulless on purpose or kidnapping Ben and Lisa instead. Everything he's done and sacrificed for them means nothing. And that's just S6, that's before Cas does anything to Sam's head. It seemed like Dean and Sam both even had moments where they understood this in S7 - Dean's "you did the best you could at the time", Sam's "you were only ever trying to help".
But speaking of Sam's head, let's talk about that for a moment, shall we? Leaving aside how OOC it was for Cas to hurt Sam after everything he's done for them - it's not just Dean and Sam who've borne bad writing these past two seasons, after all - this was a terrible, terrible blow to inflict. This was specifically doing what so many bad guys have done to the Winchesters in the past, or tried to: use one against the other. Incapacitate Sam, and Dean will be out of the game. I am in no way defending this action, I swear. It was wrong. It was wrong for him to promise to save Sam as long as Dean "stood down" - although to be fair, I don't think he thought it would ever be a question, once Sam was down for the count, making a cure a foregone conclusion - it was wrong. It was a mistake. This is the thing I absolutely get still being betrayed by and pissed at and have difficulty getting over, especially for Dean. I mean, it's Sam. (I just want to point out something that people apparently miss, which is that Cas brought down a wall that was already crumbling and would have fallen completely if Raphael's plan succeeded and Lucifer rose again. I'm not defending his action, but with the treatment of it in fandom and the show, you'd think it was less 'betrayal in desperation' and more 'Cas personally tortured Sam for all that time'. There's a difference there in what is appropriate to be pissed about.)
But the thing is, Dean and Sam both have done terrible, hurtful things to each other in desperation. Dean made a deal to sell his soul which nearly emotionally destroyed Sam, and is indirectly responsible for Sam's S4 adventures in blood because of it (NOTE: NOT BLAMING DEAN, I DON'T AGREE, JUST SAYING THIS IS AN INTERPRETATION DEAN "SELF-HATING" WINCHESTER COULD AND PROBABLY DID CONSIDER), because he was desperate to save Sam. He threw out that "not my father" line at Bobby and told Sam he didn't think Sam could resist the devil in Point of No Return because he was desperate and wanted them to hate him before he said yes to Michael. Heck, he called Sam a monster in When the Levee Breaks because it was the below-the-belt hit that he was desperately hoping would knock some sense into Sam. And in the same episode, Sam nearly beat Dean to death in that motel room before he left with Ruby to fight Lilith, said some truly hurtful things about Dean being too weak to stop the apocalypse and not knowing Sam, and left Dean lying there. Desperation makes you do awful, awful things and hurt the people you love because you feel like you have no choice. They both know this and get this, more than normal people would for that matter. It should not be a dealbreaker when it comes to Cas. It wasn't a dealbreaker for either of them, and Cas is - by Dean's own admission, albeit late - family. Same season, Dean saying to Bobby, it doesn't matter when it comes to family, you forgive and forget. The incident in Omaha Bobby's talking about, that Dean says Rufus should have let go? Bobby got Rufus' daughter killed. It's in Bobby Singer's Guide to Hunting. But it doesn't matter, according to Dean, because you forgive family. But not Cas, apparently. How is that fair?
And the second Cas was back to himself after Godstiel, he was apologising. He was saying if he still had the power he would fix Sam's wall then and there. There was nothing, not all the dead angels, not the dead humans, nothing, he regretted more than hurting Sam (and Dean by association). He sacrificed himself - or tried to - by throwing the souls back into Purgatory, and then when he survived he immediately promised Dean that he'd redeem himself. He dies before he can make good on that, of course, but then once he regains his memories after Emmanuel, again his biggest guilt is that he can't fix Sam. He genuinely believes he should be dead because he can't fix Sam. And then he goes above and beyond by taking all that pain and suffering out of Sam's head and into himself, with no idea what it'll do to him and not caring at all because he wants to badly to make things right for Sam.
And then Dean especially spends this episode not giving two shits about Cas now that he's crazy. Yeah, I know all the theories about Dean just being self-hating over how crazy Cas is, and hating how this Cas isn't his Cas any more, I've heard all the justifications and I can see them, but -- he spent that board game yelling at Cas and telling him he's not sorry enough. He says over and over that angels are junkless dicks who don't have the ability to care. He banishes Cas along with the other angels without so much as a blink and just lumps them together in the one-liner afterwards about all angels back to their corners. He blames Cas outright for everything they're going through, when nobody knew about the Leviathans beforehand, and - once again - Cas had been just trying to protect them. He and Sam barely move at all to defend Cas when he is being beaten to death in front of them and is about to be stabbed. They don't say anything, don't ask if he's okay after Meg saves him, frankly don't even look particularly bothered while it's happening. Sam says they'll do everything they can to fix him, but they don't even attempt to stop him flying away or ask him to stay with them to actually do that. Inias, Hester at first, and even Meg all showed more affection for Cas than Dean and Sam did. And Cas slaughtered so many angels in heaven, but Inias forgave him so immediately that he wanted Cas to come back to the garrison with him. Angels, who supposedly can't care and don't feel emotion, are more forgiving and kinder than the boys. Meg, a self-serving demon who hates angels by definition of what she is, showed more patience and snarky affection than they did.
Basically, Cas is supposedly family, but the boys - Dean more than Sam - are treating him like, at best, a tool that stopped working right, and at worst like some Dick Dastardly moustache-twirling 2D 'bad guy' who never did a single good thing for them and refuses to clean up 'his' messes. He gave everything for them. He died repeatedly for them. He left his own family for them. He compromised everything that he is for them, over and over. Then he made one truly awful mistake while trying to do the right thing, and hurt people he loves. For that, he's villanised and degraded and nobody gives a shit about him any more. Everything he ever did in the past is wiped out by that one mistake. Meanwhile Dean and Sam are allowed to acknowledge the good intentions behind their same situations in the past and forgive each other, however hard the road.
This isn't what the show taught me about family. Family don't end with blood. Cas earned his place on the Winchester family bench over and over, and the Winchesters are not so flighty and free with the family label as to take it away once given. This isn't what the show and the Winchesters are supposed to stand for. This isn't what the Dean and Sam I know from season 1-5 would do. This is out-of-character bullshit, and it's unfair.