Yes, Castiel's attempt on Sam's life - and that's what it was, he knew for a fact that Sam could die when he destroyed the wall - is an absolute no-go in Dean's book. There are other differences, too. When Sam made his 'deal' with Ruby, he was a drunken wreck, mad with grief and survivor's guilt, but when Castiel made his deal with Crowley, he was of a sound mind. When Dean finds out about Sam's activities with Ruby, Sam compromises. He gives it all up and tries it Dean's way and only if that doesn't work, he resumes his activities with Ruby. Castiel, on the other hand, flat out refuses to even listen to Dean's attempts to reach out to him, he never even considers Dean's proposal to work together, he just demands subordination. Sam always used his powers for good, even Cindy's sacrifice was done with the intention to save the world. Soul-empowered Castiel goes on a killing spree of massive proportions out of petty disappointement with his father. Overall, the situations are so very different, and I feel that, added together, Castiel's transgressions are so much more grave than Sam's. It boggles my mind that so many people feel that Castiel's actions can be handwaved somehow.

Sorry for the ramble, lol.