He knows that what Sam's doing doesn't sit right, and he's trying to get Sam to SEE that. But he also knows that if Castiel makes good on his threat and kills his brother? Dean's going to do everything in his friggin' power to try and stop him, even if that means his death too. Because, you know, the lovely Winchester codependence.

Dean's not buying it. He says that if Ruby's knife can't kill it, then they're pretty much fucked.

Well, if it's a Bela lives!verse, she still has him. If it's a Bela dies!verse, it got left to her neighbor in her will. *nods*