While it is true that Castiel and Sam's past situations are not perfect analogies, I think the core point rests in this: by now, Dean should have known enough to be able to understand when someone is backed into a hopeless corner and makes a bad decision - even if it results in real damage.
By the end, both Sam and Dean together are responsible for releasing Lucifer, after all, and all kinds of crap went down because of that. They both have to live with that.
Sam and Dean making some attempts to offer assistance to Cas doesn't seem much different than either Sam or Dean offering to help one another when there's trouble brewing, only for one to push the other way for a while. Again, Dean should have had much experience with this and been wiser.
For his part, Cas' real mistake wasn't in a desperate alliance - everyone in Supernatural has played that card - but in being caught off guard repeatedly by Crowely abusing the terms of their deal and committing atrocities that Cas wouldn't have approved of. "Crowley went too far" was the repeated mantra. This demonstrated an in-character blind spot Cas has: he was being written correctly as the angel whose point of view was actually too large for its own good. He was so certain he could control Crowely in the end because "he was still an angel and Crowely just a demon" that he didn't see the significance of how Crowely would (and did) run wild up until the bitter end when Castiel pulled his plug.
In a sense, the whole situation is an authentic tragedy by classical terms. The impasse was destined to happen due to everyone's efforts being undermined by their own nature, against their own best intentions.
Cas didn't come clean with the brothers or propose his plan openly because he thought he could control Crowley and was distracted by too many things his position required.
Dean's own hypocrisy wouldn't allow him to see around his outrage when he found out about all this late in the game.
So, as seems to be the case with a Supernatural clusterf**k, everybody goes "my bad" after the crap's already gone down and the world is ending. Again
