Close your eyes.
Sam gave up hope. He'd tried everything to save Dean, every trick, spell, potion, threat and prayer he could think of and it all came down to this – Dean was going to kill him.
Even now, he'd tried every last logical, emotional, or far-fetched argument to get Dean to want to live, to survive, to beat the Mark.
"You're a good man," he said, begging Dean to believe what Sam had known his whole life. "The real you is a good man."
The last thing he tried was attacking Dean, getting into a knock-down and knocked-down-again fight, hoping, desperately hoping, it would make Dean fight to save himself.
But all it did was show Sam that there was no hope. The good man inside of him was gone. There was no hope of saving Dean from the mark. There was only the hope of saving the world from Dean.
"All right, do it."
So Dean took the scythe from Death and stood over Sam.
"Close your eyes."
It sounded like an order, but Sam didn't follow it. What did it matter now?
Then, softer, like a plea, "Sammy, close your eyes."
And Sam was seven years old again, and a frothing, feral, mangy cat had him trapped against a tree in a wooded parking area. Dad was taking aim at the thing with his gun and Dean stood beside him, where Sam could see him. "It's all right. Dad's got this. Just close your eyes. Sammy, close your eyes."
All his life, when there was something too big or too scary or too overwhelming Dean would comfort him, reassure him, protect him – 'Sammy, close your eyes' – even when it was the only comfort and protection he could offer.
Sam realized that good man wasn't gone after all. Maybe he couldn't save Dean from the Mark right now, but he still had to try to save him.
"Here," he said and pulled the pictures of Mom out of his pocket and offered them to Dean. "When you find your way back, let these remind you how to be a good man."
And then the blade fell and Death was gone and Dean was cured.
Sam closed his eyes.
The End.