User blog comment:ImperiexSeed/The oldest being, archangel or leviathan?/@comment-190.92.7.204-20150612220839/@comment-3045936-20150613235239

It's speculation regardless. When Sam was about to kill him he said Lilith would kill 100+ if she could. Alastair might have owned Castiel at the time, but he was just an regular angel at the time. Not a Seraph like was season 8 and after. Every time a demon killed an angel in the series it was with an angel blade or a weapon made from a blade. Crowley even said Cain was the deadliest demon alive and telekentically tosses Castiel, the only demon to do so. The demon killing knife that at least affects to kills all other demons, didn't hurt him or suprise him.