If you send Soren to jail, you discover that he did it. He was the one following Magnus around the ship, and he apparently knocked him unconscious just before the ship pulled into the harbor, after which he took him to the ice caves in his dinghy. I'm not sure why, since I don't really see that destroying the ship forwards his interests in any way. But with the rage he's built up, maybe Magnus said the wrong thing and Soren just snapped.