User blog comment:ImperiexSeed/The oldest being, archangel or leviathan?/@comment-25291610-20140626230522/@comment-24834479-20141029171955

You do not have your facts straight. Castiel did not kill Raphael with the souls of the Leviathans. He used the souls of all the monsters in purgatory, so that he had enough power to kill him. Yes, the leviathan were absorbed by Cas also, but they were not the reason he could kill Raphael. And there is no reason to assume that Archangels can kill Leviathan. We know that Leviathan can kill Angels, as it has been stated and shown in the show. Nowhere in the show does it say that archangels are excluded. That means that leviathan can kill archangels as well. I think you should look into it.