I believe the narrator is insane. To take a guy to the vault and chain a wall then builds a wall around him.   To even plot up such an idea you have to be nuts. As far as I can tell he did nothing that substantial that it does not mention it in the story. So the narrator must have so up session with inflicting pain on people and causing them harm in the most insane and bazar way. Also to no so much about one man he had not just met him one time. He must have been observing him for a long time.