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So this was how it would end. Sam had always wondered what it would be that would finally finish them, how they would finally break the cycle. Of everything that crossed his mind, it was never this. Even after everything Dean had done in the past year, Sam never thought it would come to this.
Dean just watched as he approached, sat on the steps with a beer beside him. He looked the same as he always had, for a moment Sam almost believed he had his brother back - and then he saw the eyes. The black eyes that had watched so much death. His brother sat before him, but he died a long time ago. Dean had died the moment he grasped Cain's arm. Hell, Dean was doomed from the moment their mother had started to burn.
But Sam could never really let him go. There was always some chance, some spell or deal, that could bring them back. Even when Sam had thought it was the end, possessed by Lucifer and hooked on Demon Blood, there was always a way. That was why Sam had spent the past year doing what he had been, travelling the world and making deals with demons and angels alike; he knew that if he got close enough to Dean he would lose his resolve. He wouldn't be able to kill his brother, even if he could overpower the being he now was.
Even now, standing on the other side of the square, Sam yearned to speak to him. To ask for one final explanation. Most of all he just wanted to know why. What motive did Dean have? To kill all those people, to kill Castiel?
The memory shot through his mind like a knife. That was the moment he had given up on saving his brother, the moment he started searching for a way to kill him.
And Sam had found a way. Crowley, godforsaken Crowley, was the one who had it in the end. And Crowley was the one who hid the fact that the Colt was now in Sam's possession.
So, rather than fighting through the demons before him to reach his brother, rather than fighting him until one of them died bloody, Sam just fired. Dean barely had time to blink before the bullet pierced his skull. He was dead in an instant.
Sam died just a moment later, as Dean's minions swarmed him. He didn't fight, there was nothing left to fight for. No heaven, just hell, and he was damn sure that was where he was going to go. When he died, his last sight was Dean bleeding slowly from a single wound to the head, amongst the masses of limbs converging. In that last moment, he looked human again.
In the end, it was only Crowley who survived to tell the story of the Winchesters.