[[Thing I wrote because I can practically hear Dean screaming it whenever he makes horrible decisions that are not his to make. Focuses mainly on first 8 seasons plus ep 9x01]]
It was horrifying, the first time Dean realized that Sam couldn't be trusted. Not that Sam wasn't trustworthy, Dean trusted Sam with his life. The problem came because Sam couldn't be trusted with Sam's life.
The first time Dean realized it, he was sixteen years old and on a hunt with his dad. Sam was supposed to be in the car. He was supposed to wait for them to come back. Dean's little brother had done all of the research, his clever little brain leading them to this small spot in the woods to hunt down the monster, but he was supposed to stay back. Stay hidden. Stay safe.
But he didn't. And when the monster reared up and almost split Dean in two, it was Sam who jumped between them. Sam's skin that split. Sam's blood that spilled all over the forest floor. Dad appeared like a hero coming out of a story, killing the monster and torching the corpse while Dean frantically held his baby brother together with his own bloodsoaked jacket.
Sam ended up being fine. Three nice scars adorned his chest, the very first ones he'd earned. Dad celebrated by buying them all drinks, even letting Sammy have a little beer. Dean hadn't been able to breathe. Sam had put himself in danger, he'd allowed himself to be sliced open. Sam's choices could get Sam killed.
The realization was only reinforced as the years went on. Sam went off to college, and for the first time in his life, Dean wasn't able to stop him. He had to watch his brother walk out the door. His eyes hard, and stubborn, and underneath completely broken. And maybe Dean hadn't let himself see that heartbreak at the time. He'd been too angry, and too hurt to see Sam's departure as anything other than complete abandonment. But Sam had deliberately left the safety of his family. He put himself in danger just so he could have the illusion of being normal. But the joke was on him. Sam couldn't be normal, anymore than Dean or their dad could be normal. And when the monster came for Dean's little brother, it was a miracle that it didn't go for Sam. If Dean had been slower . . . if the target had been Sam's death and not Jess's . . .
The thought was too horrible to think about.
It was Dean's job to protect Sam. He failed when Sam chose not to kill Jake and got stabbed in the back. Sam didn't put his own safety as the priority. He chose mercy. He chose to spare the other boy's life.
And it got him killed. And Dean had been unable to stop it.
Sam couldn't be trusted with Sam. He jumped. He jumped into that pit with Lucifer riding shotgun. And Dean agreed to it. He agreed to it. He let his brother jump. He let his brother die. He agreed to a plan where the winning outcome would be Sam being tortured in hell forever.
That was the end. The breaking point for Dean Winchester. The moment that he stopped pretending that Sam was allowed to make decisions about Sam. Where he stopped pretending that he would respect Sam's wishes if it meant those wishes would take Sam away from Dean.
Dean was done. When his brother walked back into his life, he was a soulless dick. And maybe getting his soul back would hurt him, but Dean's baby brother was burning in hell, and Sam couldn't make good decisions about Sam, so Sam didn't get a say whether or not Dean shoved that soul back in.
And Sam didn't get a say as to whether he knew about anything that might trigger his hell memories. He didn't get to decide!
And yet Sam did decide. He poked at those memories, he tried to figure out what his soulless self had done until he'd had seizures. If Cas hadn't broken his wall, Sam probably would have done it himself. Dean hates that he knows that. It makes hating Cas a little easier, because otherwise he'd have to be angry at Sam.
And yet somehow it was still Sam that ended up saying that incantation, taking on the trials to close the gates of hell. Somehow, Dean couldn't stop it. Dean let Sam take on that burden, and it almost killed his brother. Sam would have died for the world, again.
Faced with the same choice he had made three years earlier, Dean found that he couldn't do it. It wasn't even a question, there wasn't a moment where he even considered it. Naomi told him that if Sam finished those trials, he was going to die.
"Take me to him now!" Dean screamed at Cas. He should have never left Sam. He should have stayed by him. He should never have let Sam take on the trials in the first place.
This wasn't their mess to clean up. Sam didn't owe the world anything. Dean had to stop him.
And for the first time, for the very first time, Dean succeeded. Sam put down his hand and let the power to close the gates of hell drain out of him. He lived. Sam Winchester would not die to protect the world.
So when Dean found himself staring at his brother's pale, still form lying on that hospital bed; when he heard his brother beg Death to ensure that if Sam died, he would stay dead, Dean knew.
Sam couldn't be trusted with Sam. It was Dean's choice. Because Dean was the only one who cared if Sam lived or died.
And Dean chose.
Wrong.