It is Macbeth's wife who causes Duncan's death. Macbeth was loyal to the king and he did what he wanted. The only time Macbeth killed was while he was in battle. Therefore, the witches also provoke the murder by giving Macbeth his hopes by telling him that one day he could be the king, believing that he is capable of greater things. With this new desire, Macbeth didn't know what to think and he wrote a letter to his wife. He told his wife everything the three witches had told him. When Macbeth told his wife about it, Lady Macbeth put ideas in his head that changed him. They simply “unleashed a monster,” even though Macbeth did not know that part of his monster was attached to his wife. His wife is the motive that encourages him to do what he did and that is to kill the king. Macbeth's ambition to become king led him to kill Duncan (the king). Additionally, the argument Lady Macbeth uses to convince Macbeth to commit the murder, telling him not to be a coward, not to say later that he "could have been" when he could have been king. She tells him to be a man and go get what he wants…. Make him commit the murder of King Duncan. Also, the wishes made him become a "monster" after the witches.