Comment on the supernatural element in "Macbeth."
We must remember that for Shakespeare and his audience, the "super" natural, properly speaking, did not exist. Witches, demons, and ghosts were all integral parts of God's creation and all assisted in working out the Divine plan, even though they might in their delusion believe that they were impeding it.
These elements in Macbeth thus serve to test the morality of the protagonist and to demonstrate divine power by frustrating mere man's attempts to defy destiny. The announcement made by the witches to Macbeth at the start of the play is morally neutral -- as Macbeth realizes, if it is indeed his destiny to become King of Scotland, he does not need to act to realize that destiny.
Macbeth: [Aside] If chance will have me king, why, chance may crown me,
Without my stir. (I.iii)
Nevertheless, urged by his wife, he overcomes his scruples and murders the king, despite the warning of the bloody dagger in the air, and in the midst of a portentous storm that clearly indicates Nature's outrage (II, 4).
Ross: ...by the clock, 'tis day,
And yet dark night strangles the travelling lamp:
Is't night's predominance, or the day's shame,
That darkness does the face of earth entomb,...
The throne secured, Macbeth then wishes to go further and do what the witches had clearly said he could never do, keep it in his own family forever. He attempts to use his petty human intelligence to defy divinely ordained destiny, only to be frustrated first by the predictions of the witches that Banquo's line will rule both Scotland and England (Macbeth: "...shall Banquo's issue ever / Reign in this kingdom?" (IV.i)), and then by his hasty misinterpretation of the predictions about how his castle might fall and who could kill him.