|“||This isn't you!||”|
Sam Winchester was a hunter from an alternate universe.
He was an alternate reality version of Sam Winchester.
After Sam Winchester shot God in Moriah he started having visions of one of the brothers killing the other which was revealed later to be God's memories from another world's Sam and Dean which Sam started seeing because of the link that The Equalizer created between Sam and God. In this universe, Dean never got rid of The Mark of Cain and he turned into a Knight of Hell. Demon Dean hits Sam, causing him to fall from the catwalk, and then Demon Dean teleports to Sam, and while Sam tries to talk him down, Demon Dean grabs Sam by his neck, strangling him and lifting Sam off the ground, Dean's eyes go black before he takes The First Blade and drives it into Sam, killing him.
In The Trap, God reveals that all of the visions Sam has seen are really God's memories of the deaths of Sam's and Dean's from alternate realities.