At the beginning of the play, Macbeth has just proven himself on the battlefield, and then he meets the three witches. They prophesy that Macbeth will be king, and this starts him thinking about how that can happen.

For King Duncan's murder, Lady Macbeth plays a huge part in getting him to go through with it. She questions his manliness, and he feels he must kill Duncan to prove himself to her. The prophecies of the witches also help to convince him that he must do this.

After the murder of the king, Macbeth allows his ambition to control him, and this is the beginning of the end for him. When the witches tell him that "no man born of woman" can harm him, he feels he's invincible. His love of power becomes greater than his love for any person. This is seen when he can't be bothered when the doctor tells him Lady Macbeth is very ill.

Once the prophesies of the witches come true, Macbeth becomes so confident that he thinks he can get away with anything. He has Banquo killed. He kills Macduff's family. After he has killed once, it becomes easier to do it again. The witches trick Macbeth into thinking he can't be hurt, he is ambitious, and by the end, he is totally insane with power.

Macbeth struggles with his conscience quite a bit in Act One, arguing with Lady Macbeth and with himself. In the end, however, he is spurred on by two things: one, the assurance of the witches. The confirmation of their prophecies that appeared in the form of announcing him Thane of Cawdor has made him believe that to be King is both his right and his destiny. When Malcolm is announced Prince of Cumberland, Macbeth does not feel that he has hoped in vain - instead, he feels as if he has been cheated. It is his rightful title. He has just won a war for the king. He deserves the highest award. To be denied is not an option, and he must "o'erleap" that step.

In addition to his prophetic assurance, Macbeth has the assurance of Lady Macbeth, who calmly and rationally provides him with a plan to kill the king and come away with clean hands. She explains her idea of using the servants as the scapegoats, and tells him plainly "screw your courage to the sticking point and we'll not fail." Macbeth is easily controlled by her.