Macbeth heard the prophecy from three witches. When Macbeth received the prediction that he will become thane of Cawdor and then it actually happens, his character changes from a nobleman to a villain (with the help of Lady Macbeth). His ambition is what leads him to killing Duncan and so when the witches told him that he would become king, he intensified his efforts to kill Duncan so that he would become the king as soon as possible. This desperation can be seen in Act 1, scene 4,