Reaction fic. changed slightly because my mind is piecing it together differently.
Sam can't move. All he's seen is Metatron shove a blade into Dean, suddenly Sam's running forward to try to help his brother. 'Castiel! I need you.' He thinks. No response. Sam looks around and charges at Metatron, but he's gone. Instead of searching for him, he turns around and goes back to tending to Dean.
"Please Dean, hang in there with me, please, I can't lose you." Sam begs, trying to keep pressure on Deans wound, but it's not working because his hands are soaked in blood and Dean's about to die. "Dean, please!" Sam cries, to no avail because his brother's gone limp and holy shit, Dean's dead.
"DEAN!" He cries, holding onto his brother's limp body. "DEAN!" He cries again, but it doesn't work because Dean won't wake up, and he's alone. He leans over and cries. Letting out a heartbreaking scream, he leans back over and hugs Dean to his body. "Dean, I'm so sorry." He sobs. "I'm sorry that I've failed you as a brother." He adds.
He sits and cries for a long time because Dean's dead and he didn't save him and what can he do? CROWLEY. He picks Dean's lifeless body up and carries him. When he returns home, he lays Dean down on the bed, and goes into a seperate room, what he doesn't notice is Crowley standing in the shadows.
When he's sure Sam's out of ear shot Crowley comes out and begins to talk to Dean's lifeless body. He sits down, continuing to talk, sighing he stands up and takes out the blade, putting it in Deans hand and smirking. "Feel that Dean Winchester? That's life." He said, and in an instant after he's done talking, Deans eyes snap open and their pitch black. Crowley's done it. He's taken Dean Winchester and turned him into a Demon.
He's got his weapon, now to use him.