MacaBeth has three reasons not to kill the king. First, killing Duncan may come back to harm MacaBeth in the end. Second, the king is currently in MacaBeth's care and MacaBeth is his host. Third, Duncan is a noble man and many would miss him. MacaBeth's only reason for wanting to kill Duncan is his ambition to become the king himself. He decides not to do it. However, he is eventually convinced by Lady MacaBeth to go forward with killing Duncan when she questions his manhood.