Ruby dies, turning to dust in the blinding light, and Sam is viciously glad even in his misery.
Then he sinks into the light and there is infinity surrounding him. All that ever was and ever could be. Then he sees what the world could have been without humanity and there is bitter regret at the loss. There are low notes from a flute and the high screeching ones of an organ and soft, barely audible ones from the wind rushing through reeds and over a pond that's teeming with life. Sam never knew what that sounded like and he's grateful that he knows now, even though it's breaking his heart.
He holds out his hand because he wants to show Dean and he cries out when he remembers that Dean isn't with him, and never will be again.
Dean doesn't understand, he's told and it sounds like it's coming from within him, from his heart and also from every part of the world. He shakes his head, Dean was right, he ruined everything and he should have been killed.
He sees the beginning of the world, not quite the beginning because only God and Death have seen that. Nevertheless, he sees the stars blinking into being and it awes him that the bare pieces of rock in front of him will one day be the Earth. He sees a hundred million species in a few hundred worlds and he sees them thrive. He sees the Earth rest in peace.
Then come the humans, the precursors to homo sapiens sapiens and the beginning of the end.
Of all my siblings, I was the least made for war. Death was my domain, and death is inevitable. But I saw that death would come before their time to all of my Lord's creations if he favored this one. Sam sees then, that the destruction of everything beautiful is not yet unavoidable. If the world is cleansed of every selfish person that cares nothing for it. Then he thinks that Dean wouldn't want this.
Dean doesn't understand, he's told again, and there is a deep empathy there. Sam sees another pair of siblings intertwined since their birth argue bitterly and he feels the pain when they jerk apart and it burnt, oh it burnt. They had forgotten (if they ever knew) what Grace belonged to who, and who was supposed to sing which part of their song. He clings to the hope that with enough eloquence his brother could understand and Sam doesn't know whose hope it is.
Then he sees the world they would create where everything was just and right and he (they) think that Dean (Michael) would understand when they see this. That they only needed to see the proof that it'll be a better world. And forgiveness will come and brotherhood.
We must save everyone; there is no one else to do it. Sam wants to, but he thinks he's made enough decisions that have led nowhere good and as he hesitates, he is asked, with curiosity, But they brought you here, and you are needed. Is this not sufficient?
Sam can't remember why it isn't. Then Lucifer grips his arms and Sam looks at him and sees the most beautiful, the most honest creature he has ever known. He is asked, Will you not pity me, and help me get back my brother?
And Sam says, "Yes."