The introduction of the supernatural world into the play Macbeth and that competes with the Chain of Being, the natural order of things. Thus a dichotomy is set up which adds to the internal conflicts within Macbeth. More than anything, however, the element of the supernatural introduces to the play a certain ambiguity which allows Macbeth's "vaulting ambition" to rationalize his actions. For instance, after the first prediction of the three sisters, Macbeth tells himself,
If chance will have me King, why, chance may crown me,
Without my stir. (1.3.144-145)
I would say you should look at the witches' predictions and see how they encourage Macbeth to act. Look at how the first predictions encourage him to kill Duncan. Look at how they make him want to kill Banquo and his son and Macduff's family as well. Look at the second set of predictions and how they make him feel like he is invincible.