From the beginning of the play, the witches are disrupting the natural order. In the opening scene, the three witches chant: “Fair is foul, and foul is fair” (enotes etext pdf p. 8). The witches plan to meet Macbeth later, even though he does not know it. The witches interfere with fate by messing with Macbeth and putting ideas into his head from the very beginning. When they give their prophecies that Macbeth will be Thane of Cawdor and king, and Banquo’s sons will be king, they set into motion a large chain of events. Macbeth begins to believe that he is fated to be king.
If chance will have me king, why, chance(155)
may crown me
Without my stir. (p. 16)
Unfortunately, Malcolm is named the king’s successor.
When he tells Lady Macbeth of the witches’ prophecies in a letter, she urges him to be king. At first he has second thoughts, but then he has a vision of a dagger covered with blood.
I see thee yet, in form as palpable
As this which now I draw.
Thou marshall'st me the way that I was going,(50)
And such an instrument I was to use. (p. 27)
The unnatural vision is a direct result of the ambition and madness the witches stir in Macbeth. The murder of Duncan sets in motion a chain of events that leads to Macbeth’s doom. Macbeth has visions of Banquo’s ghost after he killed him. He decides to visit the witches again, to get more advice.
He is presented more visions, including the prophecies that he cannot be killed by a man of woman born, and that he is safe until Birnam wood will come to Dunsinane. Yet they also tell him to beware Macduff. All of these predictions continue to influence Macbeth’s fall, as his reactions to them seal his own fate. He behaves in increasingly irrational ways until his final fight with Macduff, who wins.
Page numbers are from the enotes etext pdf that can be found at the first link.