Barabas “laugh[s] and jeer[s]” when he reads the letter because he is happy that Ithamore has betrayed him. Barabas was looking to avenge Ithamore before he even betrayed him, so he is thankful to have a reason to kill him. Ithamore is “faithful” because Barabas knew he would betray him. Ithamore is conniving and greedy, like most of Marlowe’s characters, which implies such qualities are inherent human traits.