"Do it," Sam almost whispered, as he tried to be strong for Dean's sake.
"Do what?" Dean wanted to ask, not sure if he was analyzing the situation correctly.
"You heard me, if you won't then Death will," Sam pleaded. He would do anything to save Dean, if that meant sacrificing himself then so be it.
Dean reached for the scythe, he would not let another take his brother's life. He did not want Sam's death to be painful. In all honesty he did not want Sam's death, a sign that maybe in fact any variation of Dean would always care for Sammy. But there was no other way. They had to be stopped. They could not damn the world again. This way it would be okay. This way he could save the world and send Sammy to Heaven.
"Close your eyes," he whispered. It almost sounded like a command. However, Sam refused. "Sammy, close your eyes," he begged softly. It was not the first time Dean had uttered these words.
"Sammy, close your eyes," the words Dean murmured to Sam the first time he witnessed their dad killing a monster. Dean was not afraid that Sam would be scared but rather that the event would change him. Despite what others may think, he wanted to preserve Sam's innocence for as long as possible.
"Sammy, close your eyes," he thought as Jess was burning on the ceiling. Not wanting Sam to be exposed to such horror. Dean, on the other hand, never closed his eyes as he had to be the one to face the evil so that he might protect Sam from the brunt of it.
"Sammy, close your eyes," he silently begged as the hellhounds ripped him apart on Lilith's orders. He did not want Sam to witness his death, did not want that image to haunt Sam's dreams and rob him of the last bit of innocence he had left.
Now, as he held the scythe he did not want Sam to witness the ultimate evil. As he swung, he changed his mind. He would, had on many occasions, damned the world to save Sammy.
As the darkness was released, Dean closed his eyes, hearing Sam's pleas for him to do something. "Dean, Dean," he heard Sam say. Dean knew that if they were to die they might as well die together, as warriors, as brothers, as family. He opened his eyes and let the darkness consume them as he whispered, "Sammy, close your eyes."