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Sam watches Dean plunge the knife into the kid's chest, slowly, determinedly, without hesitation or doubt.
No remorse. No flicker of a conscience.
Dean has left the building.
He can't forgive Dean for what he did, not now or maybe ever, but he can't let Dean become a monster. Sam cut Dean off, and he was Dean's only link to his humanity.
He has to pull his brother back from the brink, and he's thrown away the one sure-fire weapon he had in that fight.
All he can hope for is that it's not too late to get it back.