@ Mako, agreed, sacrificing himself to stop Lucifer was very noble and redeeming in my eyes as well and probably a high point of Sam's character development.
Dean has always been the older brother and always tried to carry Sammy forward through everything, as well as literally being his brother's keeper. However, Sam's sacrifice was a testament to taking the reigns of their turmoil and finally stepping forward into making his own decision, this time to save Dean as well as the rest of world, and arguably the largest decision made. In this sense, Sam's redemption was earned and well forgiven. This in itself was also the climax of Dean finally realizing that Sam was grown up and wasn't just his little brother anymore, but a brother all-together and none-the-less.
After all this tho, Sam's schizo-torment seems a little forced and may possibly downgrade his previous redemption, but we have to see first.
Further, thinking of it this way, if Cas is to find a way to redeem himself in Dean's eyes, it could lead to him maybe becoming human, or possibly moreso.. dying permanently. Of course, that's it if works in a similar formula (and if he comes back).
@ OWD, I feel the use of Eve as a Big Bad, was moreso meant as a distraction to what was really going on, namely Cas' deal with Crowley to overthrow Raphael, at heart meant to be good intentions.
@ Topic, I feel another deeper reason that lead Dean to act the way he is lies in his feelings towards Cas' betrayal. See, Cas started to, in a way, follow Dean's lead in Season 5 as he slowly developed human emotions as humanity was all so new to him. But because he became so attached and close to the Winchester brothers, Cas only wanted to protect and keep them safe, as well as taking in Dean's view of "family and freedom" to heart.
Cas didn't tell them because he wanted to keep them safe and really believed he could pull it off, because he felt Raphael would only bring more torment to them as well as mankind itself if the Apocalypse was restarted, however, Cas was still new to bearing humanity. In this way, he is still a child in human terms of comprehending his emotions and understanding them completely (hence Dean's referral to him as a "child" in the 1st episode).
Overall, Cas made a devastating mistake in trying to create his own path to doing the right thing within his new humanity and I believe Dean knew this, which is part of what made him so angry. He was angry and hurt over this, but also because it made him feel as if he had been betrayed, feeling offended at Cas' lack of trust in going to them for help. It's a confusing mixture really.
I hope this all made sense xD, but this is how I see it.
EDIT: Whew, took me a while to write that O_o
~ Raz
