I don’t hate Dean. I don’t blame him for pushing for Jack’s sacrifice (though it is a bit out of character) I can see why he was so angry and why he was willing to do what he was.
Remember what he said, in the 300th episode?
“...For the longest time, I blamed Dad. I mean, hell, I blamed Mom, too, you know? I was angry. But say we could send Dad back knowing everything. Why stop there? Why not send him even further back and let some other poor sons of bitches save the world? But here’s the problem. Who does that make us? Would we be better off? Well, maybe. But I gotta be honest – I don’t know who that Dean Winchester is. And I’m good with who I am. I’m good with who you are. ‘Cause our lives – they’re ours. And maybe I’m just too damn old to want to change that.”
He had begun to feel secure with their lot in life. Reached clarity, as it were. He had taken the blame off his parents and off himself and had begin to feel right with...everything. Yeah nothing was perfect at the time, with Michael and everything, but at least he was the master of his own fate. He didn’t want John to go back and change anything because he was finally alright with how everything had turned out.
And then Chuck happened. And all of that certainty and clarity went straight down the drain. He wasn’t the master of his own fate, it wasn’t free will, it was all just Chuck’s little game and he was just a pawn. All of them were pawns. Their lives weren’t their own, they were a little puppet show, and Chuck was pulling the strings and so yeah he got angry
Because finally, FINALLY, he had found the reason for all his problems. What he had summed up to as just a shitty lot in life was actually a story being played out for someone’s entertainment. All the pain and all the suffering, it was all for shits and giggles. All for Chuck’s sick and twisted game. His mom deaths, his dad’s obsessive need for vengeance, his brother’s shitty life, it was all because of him. Not some blind luck of fate...it was Chuck. All of it.
So yeah. Hell yeah, he was angry. He pulled his gun out because he was angry. He wasn’t really gonna shoot Sam. He was just showing how much he believed in the plan. How much he desperately wanted it to work and how much he wanted Sam to get the hell out of the way so that it would work. Dean needed this win, and he was willing to get it whether Sam was happy about it or not, because in the long run they would have been better off in a Chuck-less world.
Then Sam told him the reality. Sam tried to get through to him and tell him about the power play (and I mean were we really surprised about that whole Billie thing) and he wasn’t able to get through to Dean when he was in the heat of the moment (see what I did there haha). But alas he did. Because if there is anyone in the world that can get through to Dean when he is that far out of his mind with rage and vengeance, it’s Sammy. Sam got through. Sam made him pause with one single sentence, “Would you trade me?” I mean come on! As if that was ever a question. And then he had to go a mention all the gushy “you’re my one protector and the only constant in my life” and Dean was soothed out of his anger. Just like in the Last Sacrifice, Prophet of the Loss, and countless other times. And yeah it is a little overplayed but it’s a testament to all the love and trust he has in his brother that it works so quickly and so often. Because Sam is his kryptonite and there isn’t anything he wouldn’t do for him.
While I do believe that the whole “Jack’s not family” thing was wayyyyyy out of character for Dean, and that the writers totally ignored the character development they started literally two episodes earlier, an argument could be made that he was just lying to himself to make it easier. But what do I know right?