The tragic downfall of Macbeth was not determined by one single cause. It was rather caused by a combination of three dark forces: supernatural, external and internal. The three witches represent supernatural forces and dark powers behind them. Lady Macbeth is an outer force that pushes Macbeth towards the bloody deeds. Macbeth's own ambition acts as deciding power in brining him to his downfall.
The very beginning of the play indicates that dark supernatural forces will be involved. Three weird sisters are preparing a surprise for Macbeth, surprise that will eventually cost him life and the salvation of his soul. Witches' predictions play very important role in leading Macbeth to the evil deeds. Witches are the first to unleash Macbeth's "black and deep desires" by promising him crown in the near future. They trick Macbeth making him to believe that he was fated to be king by promising him the title of thane of Cawdor and fulfilling this promise.
After this almost impossible prediction becomes true Macbeth decides that he should become king as well. His royal dreams and ambition begin to take over his good side. He is convinced that "Two truths were told/As happy prologues to the swelling act/Of the imperial theme." The dark forces "win him with honest trifles to betray in deepest consequence." Not only they make Macbeth thinking about murdering Duncan; they also bring him to the decision to kill Banquo and his son by saying that Banquo's children will be kings. Throughout the whole play dark supernatural powers trick and deceive Macbeth. In Act IV the apparitions playing with words convince him to continue to walk along the bloody path by advising him to be "bloody, bold, and resolute" and to "have no fear." These predictions give Macbeth confidence to murder more...