User blog comment:EmpyreanSmoke/Supernatural Q&A/@comment-27006668-20151003004333/@comment-24834479-20151003194940

If Castiel had one soul in him, he would barely be able to do anything else than what he already can. If he had one leviathan in him, the leviathan would possess him and be able to kill the archangels.

He would have been able to kill them with the snap of his fingers.

They would probably have a shot at killing him, but he would have pretty easily killed them. He didn't want to deal with them, which is why he kept the cage.