How does Captain Lamachus lose his arm? Why does he beg his fellow thieves to kill him?

Captain Lamachus loses his arm when he sticks his hand inside a door to unlatch it, and a man nails his hand to the door. While escaping, the bandits slice off his arm so that Lamachus can run away. He begs them to kill him because he cannot steal without his hands. He thinks that he is now useless.