Sam is not his father's son but in fact the son of Azazel (the Y.E.D), but he doesn't know it. He keeps fighting the evil growing inside him, but is constantly attracted to demons without understanding why.
"You can cover whatever the hell toi want. I'm still going to kill you. Maybe not today-- but toi look into my eyes, toi son of a bitch. Because I'M the one that kills you." A look at the epic struggle between the yellow-eyed-demon and dean
