A/N - So I was eavesdropping on a conversation on Tumblr last night (what do you call it when you watch a conversation on a screen?) where the subject of discussion was what would have happened if Sam had turned into something they hunt before John died, and the plot bunny took off running.
It had taken eight months to find him. Eight months in which Dean had barely eaten or slept, completely consumed by the need to find his baby brother.
To find a demon willing to spill Sam's location, and to find Sam alone, unguarded, tied to a chair in an abandoned house in the middle of nowhere was a little too convenient.
It was a trap. John knew it was a trap, and Dean should have known it was a trap, if he wasn't so overwhelmed by finding Sammy.
Dean flew across the room to kneel by the chair, untying the ropes, rubbing Sam's arms and legs to restore circulation, murmuring promises of a hot bath and a good meal once they got out of here.
John knew it was a trap even before the monster wearing Sam's skin had hoarsely moaned out his brother's name, burrowing into Dean's embrace before meeting John's gaze over Dean's shoulder and smirking with yellow eyes.
"Dean," his father warned.
The older son glanced over his shoulder to see his father still by the door, gun drawn.
"Dad?" he frowned. "No, Dad! It's Sam!"
"It's not Sam anymore," the hunter shook his head.
"It is Sam!" Dean insisted, glancing at his brother as if to make sure. "I don't know what the demon did to him, but Sam's still in there! I'll fix him! I'll get Sammy back!"
John sighed, choking the sound down before it became a sob.
He had known, from the moment he and Caleb had realized there was a good chance that one day Sam would become something they hunt, that it would end this way.
Dean would never willingly harm his Sammy. He would never forgive John for doing so.
He would never stand aside and let his father do what had to be done.
"Dad, no!" Dean pleaded, placing himself between his father and brother, arms thrown backwards to keep Sam behind him.
"I'm sorry, Dean," John said softly as the first tear overflowed.
He pulled the trigger.
His oldest son slumped into his younger brother's arms.
"Dean!" the demon wearing Sam shouted, but Dean did not answer.
Dean could not answer.
The Sam monster gently wiped away the trickle of blood from the corner of Dean's mouth, then straightened, shifting the body over to dangle from one arm, revealing a dark stain of Dean's blood on Sam's shirt, to sneer at John.
"Too bad you can't get rid of me that easily."
Despite the way the creature continued to tenderly cradle Dean's body, it wasn't Sam.
The facial expression, the voice, the cold yellow eyes, the way it held its body, none of it was Sam.
John didn't answer. He dropped the Beretta by his side, drawing the Colt from his jacket pocket.
"I wouldn't have killed him," the demon spoke again, flicking its eyes to Dean momentarily. "Only you. Big brother would have been useful. And ... malleable."
John raised the second gun and fired again.
Sam's body jolted and seized as it was lit from within, sparks of electricity flashing in the air around him, staying upright for long enough that John considered shooting again. A low pitched whine seemed to resonate from the wound, building into roar that John felt in his bones rather than heard with his ears.
The lights and noise stopped as abruptly as they had begun, and Sam's body crumpled to the floor, one arm still wrapped around Dean.
John dropped the Colt beside the Beretta, and for the first time, allowed himself to feel the enormity of what he had just done.
He sank to his hands and knees, shuddering and wailing. He didn't know how much time passed before he finally crawled over to his sons.
He gently closed Dean's eyes, which hadn't even had time to widen in shock before his father had killed him. He brushed Sam's hair out of his face.
He prayed that if there was a Heaven, that Dean could be with his mother. That she would watch over their oldest boy for eternity, while John watched over the younger one in Hell.
He wiped the blood from their faces and pressed a gentle kiss to each of their foreheads. He pressed Sam's head against Dean's neck and laid Dean's arm around Sam's shoulders.
They looked peaceful, like they were sleeping, tangled up together as they had been throughout their lives.
John sent a text message to Bobby, just four words.
"Burn the boys together."
He sent up a prayer that his old friend would forgive John for making him do this, for leaving Bobby to clean up after John Winchester one last time.
He then picked up the Beretta, put it in his mouth, and pulled the trigger.