Prophecy Fulfilled

Can't, Won't

And then! Then would come the murder you'd never survive. The one that would turn you into as much of a savage as it did me. Your brother, Sam.
The prophecy Cain told him kept going through his head; as his breathing turned back to normal, as the effects of the mark left his system. Dean kneeled, his knuckles bruised and bloody, blood on his body and face, blood that wasn't his. Dean tried to ignore the words that kept repeating themselves in his head, but it was impossible, not when he was kneeling in a growing pool of still warm blood. Sam's blood. Cain had been right, it was impossible to ignore the mark, to resist, to not give into its want for blood. He had also been right about the other murders.
Crowley had been easy, he was able to push it off, something him and Sam had meant and talked about doing for years, it always just came up that they needed him or he got away. Sam hadn't looked at him the same since he killed Crowley, worried eyes almost always on him, watching his movements. He didn't blame him. Cas had been painful, Sam had walked into the room afterwards, asking what all the noise had been, and had frozen in place, taking in the blood everywhere; taking in Cas's dead vessel, wings coming out from it imprinted onto the ground. There had been fear in Sam's eyes. He had backed away from Dean, begging him to stop, begging him not to do this as Dean walked towards him, the Mark's effect stilling flowing through him, telling him to finish this. Complete the prophecy. Sam hadn't stood a chance, none of them had.
Dean looked over to the corner, where Sam's lifeless body was; and everything came crashing down onto him. Dean let out a cry, his sobs uncontrollable. He'd killed Sam, his Sammy. The one person that kept him going, his one reason to live. The baby boy who he carried out of a burning building, who he had pretty much raised himself, who he had swore to protect his whole life, who he loved more than anything and anyone else. He'd killed Sam. The pain was unbearable, crushing him, the grief, like he was being suffocated by it; drowning in it. Dean couldn't live without Sam, he wouldn't live without Sam.
Dean brought the blade up to his throat and dragged it across the skin.