He thinks maybe he has never been good enough.
Sam is so damn tired of trying. His soul is black, black and stained and rotten all the way to the bone and God, he hates himself sometimes. He wonders how it ended up like this: one so dark and evil, forever destined to destroy the world, and the other so blindingly pure he nearly burns himself out, beloved of God and angels. Dean is the Abel to his Cain, the Eve to his serpent. The Michael to his Lucifer.
He's so damn tired. So tired, but he can never sleep because the Apocalypse is coming and his dreams are filled with insidious, silk-shimmering smiles, Lucifer and the cold grip of fate and dread, always dread, because the only thing he ever wanted was to be good and he doesn't know how he got so hopelessly turned around, so inextricably tangled in death and blood and madness.
Dean grows more distant by the day. He does not belong to Sam any longer. Sam knows this by Castiel's handprint on his arm, by Michael's voice in his head, by the days they spend in the car that are never the same as they used to be, never as easy and familiar and effortless as they once were. Sam is something different now, tainted by blood and demons, and Dean is changed—he is the righteous man, the angelic vessel, the one destined to end this nightmare. His halo is rusty, but is is a halo nonetheless. He has the power to save Sam or to destroy him, to stand by him or to kill him (and sometimes he wishes Dean just would because he knows he deserves it, he does).
But even as Lucifer twists his tongue in velvet lies and wreaks destruction upon the world with his own hands, Sam knows Dean will never kill him. Dean, he thinks, is far too noble for that.
Sam is broken.
He knows this. He accepts this. In cobweb shadows at midnight, he watches Dean in greasy motel rooms, studies the familiar lines of his brother's face and tries with all his might to figure out how he got so lost, because Sam went to Stanford once and he used to be able to understand Sophocles and Plato and write essays and solve equations and he should be able to do this, to figure out which precise point on the X-Y graph of the A to Z of his life marked the beginning of the end.
But he was never all that good at graphing, anyway—so he keeps his silent vigil, every day sinking deeper into his ruined mess of a soul, and wishes Dean's light could make him whole again.
FIN.