This story was just a little plot bunny I had, and decided to write it to try to overcome my writer's block—think it just might've started to work, too…
'Always Been Dad's Favourite'
He merely flinched as he witnessed the bullet speed toward his brother off of his own gun; straight through Dean's flesh and embed itself into his heart, eldest Winchester flailing before hitting the ground with an almighty thud. He merely grinned as Dean yelled out in pain, a small smirk across his face, because this was what it was all about. In the end, it wasn't killing Dean that mattered; it was the pain that he suffered before he finally sank back down into hell.
Where Sam knew he belonged.
He'd finally meet him back there, one day, he guessed. Once his rampage was over, when the demon no longer had a use for him. But until then, it was out of his control. The only thing he knew was that the elder brother, and all of the other hunters that hadn't shown him his demise when they'd had the chance—Bobby, Ellen, Jo, even Ash (although he suspected that the latter might have tried), wherever he was. He'd find them all, and they would subsequently pay. Sam loved the sheer simplicity of this life—of what he had become at his brother's hand. Ex-brother, because Dean was lifeless on the warehouse floor beside him now.
Sam's lips curled in disgust as he looked into his brother's dulled eyes; HOW could he have done this? It was Dean's fault, not Sam's, after all. He hadn't been there in time to save dearest little Sammy from the big-bad. Hadn't been there in time to watch his kid brother succumb to the demon's powers; almost as if he were in a Harry Potter book. Hah.
What little amount of memories that hadn't been hazed in the process of transformation were now brought to Sam's mind, and he could do no more than laugh. How stupid could Dean have been? He should've seen it from the beginning—Dad should've seen it from the beginning, instead of towards the end. Another successful kill, really, because Sam doubted that his father would've given his life for his eldest son's had Sammy not been in the mix. The demon was right—Sam had always been John's favourite.
He reached a mud-clad boot to Dean's side, overturning the body with the toe of his shoe, sadistic grin finding it's way back on to his face as he pulled his other boot free of the sticky mess of blood on the floor by Dean's chest. He shook his head, aimed another round into Dean's skull before he knew he was dead; just as they'd done to the shtriga oh-so-long ago. Another bullet, and Sam was on his way back out to the Impala, leaving behind him the mess of a brother that once was, a hand gun, prints wiped clean just as Dean had showed him how, and the pungent smell of death.