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.