Sam Winchester bolted upright in bed, gasping for breath, his hair plastered to his sweat-soaked face. He scrubbed his hands over his eyes, hard enough to make stars burst in his sight, as if he could get rid of the images still burned into his mind in sickening Technicolor. Blood and death -- screams -- the last survivor, falling broken onto parched ground --

"Sammy?" Dean had heard Sam's rude awakening, and his voice was low with concern. He flipped on a lamp and stared at his brother with sleepy but worried hazel eyes. "Hey, man, are you okay?"

"Oh, God." Sam gave a slightly hysterical laugh, still rubbing at his eyes. "Oh, God."

"Hey, hey." Dean was out of bed now, and although his expression hadn't changed much, his brow was wrinkled slightly in concern. He gently raised Sam's face to get his brother to look at him. "Nightmare? Vision?"

"Vision...I think. It felt...it didn't feel like a nightmare." Sam looked at the ceiling beyond Dean's face; couldn't bring himself to look his brother in the eye. "God, Dean," he said softly. "It was...it was..." He stopped again, feeling like a broken record, knowing there was no way he could possibly express the fragmented horror he'd just seen.

"Yeah?" Dean prompted, in that soft voice he used when he was really concerned, that voice that could almost make Sam forget what a smart-ass he was most of the time.

Sam grabbed his brother by the shoulders, and finally looked straight into Dean's eyes, saw his brother almost flinch at the intensity of Sam's gaze. "Dean," he said, low and urgent, the words tumbling out now, "you have to swear that you'll stop me if I turn evil. Swear it."

"Sam--" Dean said wearily, that all-too-familiar low ache creeping back into his voice.

"No, Dean, listen to me. The vision -- it was a vision, man, not a nightmare -- it was...it showed me the future. I don't know how far off, but man, it was screwed up. The Demon was winning, Dean, he was destroying the whole world and I...I was serving him." I was killing people for him, I was ripping children to pieces in front of their parents, and there was so much blood...

"It won't happen, Sam. I won't let it." Because I’m your big brother, and I kill all the monsters that could hurt you, even the ones inside.

"Dean, please." Sam's voice had a telltale tremor now, and dammit he didn't want to cry but he had to get through to Dean on this. "If I...change...you have to promise that you'll kill me. Man, think about it -- if there was anything of me left, I wouldn't want to live that way. I wouldn't want to be a slave to the Demon." I wouldn’t want to watch myself rip people apart with my mind. "You have to swear it. Please."

Dean's eyes looked hollow and not-quite-alive and Sam wanted to cry more than ever, but after a long moment in which the fate of the world hung in the balance, Dean finally exhaled and said, "Okay. Okay. I swear, Sam. I do."

Dean's ribs ached and there was blood trickling into his eyes, but his hand, the hand pointing a pistol at his brother's chest, didn't waver. There was just enough light filtering into the abandoned warehouse that he could see the hollows where Sam's eyes should be, weren't any more. The eyes there now were still the right shade of brown, but they belonged to something else, and the worst part was that there was nothing to exorcise.

Sam -- the thing that Sam had become -- gave a small, soft laugh that sounded like Sam's laugh would sound if you put it in the deep freeze. "Come on, Dean," he said, stepped forward slightly, hands out placatingly. "You aren't going to shoot your brother, are you? Your little Sammy? We both know you can't do it." He stepped forward again, into the light enough that Dean could see his eyes, his big brown not-Sam's-eyes.

Dean's own words from months ago echoed back at him, bouncing back and forth off the inside of his skull. I swear, Sam. I do. He hadn't really believed that it would happen, not then, not when he still had his brother in front of him, all teary and girly and overflowing with compassion and concern.

This isn't Sam, he lied to himself, but when he looked at the face all he could see was his brother.

Sam took another step. He was almost close enough now to knock Dean's gun away, and they both knew it would be for the last time.

"Dean," Sam said, and his voice softened into that gentle, let's-be-reasonable tone that he always used to calm people down. "You can't kill your own brother."

Dean choked on the promise he'd made, choked on a million memories of tiny baby brother snuggled up soft and safe against him, of a little hand clutching his finger like a lifeline, of a big hand clutching his shirt like a lifeline, of Sam, Sam, Sam with his big brown eyes and his bigger heart. He choked and it came out like a sob and he pulled the trigger.

"I'm sorry, Sam," he said, and blinked away the blood and tears to kneel beside his dying brother.

And oh God, the eyes that looked up at him were Sam's.

Not the thing's eyes, but Sam's, with all the heart behind them, all the compassion, and Dean dropped the gun and gathered up his lanky little brother into his arms like he had forever ago when Mom burned to ash on the ceiling overhead. Dean held his brother and said "Sammy, I'm sorry, I'm so sorry -- "

"Dean," Sam whispered. He fisted a bloody hand in Dean's shirt like it was a lifeline. "Dean...thank you." He sounded so young, and Dean wanted to say It’ll be okay, Sammy, I’ll fix it, because that’s what I do, I’m your big brother, I’ll make it all right, you hear me? I can fix this.

And then Sam smiled, that happy dimpled Sam smile that Dean had given up on ever seeing again, and he whispered, "You...saved...me."

An instant later his eyes fixed and glazed, and the smile faded away, and Sam Winchester was dead.

Dean buried his face in his little brother's hair, and his voice broke when he said, "I promised I would."

They said it was a clear-cut murder/suicide -- the older brother killed the younger, then shot himself. With the elder Winchester's criminal record, it shouldn't have come as a surprise that he would be capable of such an act.

But the cops who found the two of them, lying in a pool of blood in the abandoned warehouse, were always haunted by the way that, even in death, Dean Winchester clung to his brother -- as if nothing in this life or the next could ever make him let go.