The page began to flicker before Sam's weary eyes.
He'd been staring too long at the garishly coloured illustration of Cain looming over Abel, blade held high, ready to sink it into his unsuspecting brother's body.
In the aftermath of the murder, Sam mused, when Abel's innocent blood had long seeped into the ground, God had asked Cain where Abel was.
"I know not. Am I my brother's keeper?" had been the indifferent reply.
Now the Mark of that first murderer was on Dean's arm but unlike Cain, Dean would never deny being his brother's keeper; he had been from the day Sam was born, and he'd never stopped, even when Sam had shrugged him off in search of his independence.
Yet Sam also knew how easily things could go downhill.
The love he and Dean bore each other, an exclusive devotion that had resisted everything that destiny had thrown at them, had nevertheless failed to prevented them from trying to kill each other on more than one occasion.
True, each time they had been under the influence of a spell or curse, but still.
Sam had tried to shoot Dean years before in an old asylum, his mind altered by a ghostly psychiatrist, and it had been only thanks to Dean's foresight in giving him an unloaded gun that the worst had been avoided.
He had tried again when possessed by Meg, luck had been with them once more when Dean had fallen wounded into the water, out of sight.
Then it had been Dean's turn. Filled with anger against Sam amplified by a cursed coin, he'd turned a gun on his helpless little brother.
They'd tried to take each other out in tandem when a siren had infected them with its poison. Dean had lifted an axe and if Bobby hadn't shown up, the elder Winchester could already have added the word fratricide to his many attributes.
Last but not least, Dean had been a hair's-breath from slitting Sam's throat before Castiel had pulled him back with his angel-powered strength.
Sam huffed wryly.
For two brothers who loved each other so intensely that they couldn't even put their feelings into words, they were pretty open to manipulation as far as killing the other was concerned!
While both he and Dean were starkly aware of the Cain and Abel saga and of its ominous parallel to themselves; in the classical Winchester way of denial, they ignored it. Discussing it would be tantamount to admitting there was a high possibility that Dean killing Sam was the end-game.
Dean knew it, Sam knew it.
Strangely enough Sam wasn't afraid for himself. He'd been killed before. Hell, he'd even accepted his demise when Death had given him the choice.
It was Dean he was concerned about.
His big brother would never forgive himself if he was the one to end Sam's life, and in the younger Winchester's opinion Dean had enough crap eating away at his soul without adding his brother's murder to the list.
The sound of Dean's foot-steps entering the library caused Sam to quickly close the page but he wasn't quite fast enough and Dean got an eyeful of Cain with a blade raised high over Abel before the screen-saver took its place.
Of course his little brother would be pulling out all the stops to try and come up with an antidote to the Mark. Dean loved Sam even more in that moment. No matter what, they were always there for each other.
But Dean wasn't in the least optimistic that Sam would come up with anything.
There was no way to get rid of the Mark.
Eventually he'd be forced to surrender to its dark power. Cain had told him so; told him he'd end up killing Sam no matter how hard he struggled against it and Dean was so afraid, terrified that he might end up doing just that.
He'd almost succeeded when he was a demon. His love for Sam hadn't been enough to stay his hand then.
But Dean knew how his brother's mind worked.
Sam would accept death at Dean's hand, ready to play the sacrificial lamb if he had to, and in the precise moment Dean transfixed his baby brother with the blade, forgiveness for his action would fall from Sam's lips with his last breath.
Dean would be the one to condemn himself to an eternity without Sam if he did that.
Ash had practically told them he and Sam were soul-mates.
As with many things, the Winchesters had studiously ignored it all these years, the idea being too touchy-feely for them to discuss it rationally, but Dean had never questioned it.
In his heart he knew it to be true.
There was something more between him and Sam than the relationship of simple siblings, there was a meeting of souls, a bond that had resisted through fights, separations, deaths and resurrections, but if he committed fratricide as the Mark impelled him, he would lose that too.
"Hey yourself, geek boy," Dean answered, schooling his features into one of nonchalance. "With all the hours you spend staring at that screen, it's a wonder you don't need bottle-thick glasses."
"Yeah. I've been meaning to get my eyes tested...sometime."
Sam glanced over at him and Dean was overcome by a rush of tenderness.
His little brother looked so tired, so worried that Dean wanted nothing more than to hug him like he'd done in the church and tell him everything would be all right, that they'd get though this as they'd done with everything else.
The only thing holding him back was his well-practised avoidance of chick-flick moments.
But this was Sam, his little brother and he was the one who had always looked out for him.
Sam glanced over. "What?"
The younger Winchester pushed back the chair and sauntered over to stand in front of Dean.
Sam wasn't sure what to expect, and when Dean pulled him in, circling his baby brother's body with his strong arms, it certainly hadn't been that.
After the first moment of surprise Sam slumped into the familiar arms, his head finding the spot between Dean's neck and shoulder, his arms curling around his big brother.
Like two survivors in the ship-wreck that was their life, they held on tight, the solidity of their brother's body bringing them the comfort that only they could give each other.
"It's okay, Sammy. One way or another we're going to get though this." Dean assured his baby brother.
Sam's muffled mutter of aquiescence vibrated against his skin and Dean tightened his hold even more.
Whatever happened, he vowed; he was never going to kill Sam. He'd destroy himself first.