Macbeth opens with the witches, and even before we know the plot, we become aware that the supernatural will have an influence on the outcome of this play- "Fair is foul, and foul is fair" (I) leaves the audience or reader with no uncertainty regarding the inevitable outcome of Macbeth and the weird sisters' contribution to its events.
The weird sisters only need to sow the seed to being forth Macbeth's, and indeed Lady Macbeth's, all-consuming obsession with ensuring that the prophesies come true. There has been much discussion on their influence over Macbeth and any responsibility they hold for his downfall due to this "vaulting ambition." The scenes in which they are involved are short and full of impact with a rhyme scheme that adds to the pace and intensity. When the witches speak, everyone remembers!
Hectate has no doubt as to the weird sisters' contribution and is only angered by the fact that they failed to consult her - "the close contriver of all harms." (III.v)
Macbeth is clearly responsible for his own actions but it is the witches and their prophesies that goad him on and make him believe in his invincibility. The witches know that Macbeth is vulnerable to them and Hectate is adamant that they will take advantage of this fact.
"...security is mortals' chiefest enemy." (III.v)
The witches create confusion in Macbeth in their display of
frustrating duplicity .... They are interpreted variously as custodians of evil, spinners of the future
and in Macbeth's own belief, determiners of his rightful place as king. Early on, he is aware that his ambition "o'er leaps itself" and his momentary realization that what he is doing is heinous is not enough to stop him, especially with Lady Macbeth's encouragement to 'be so much more the man."
The witches continue their manipulation of Macbeth " Double, double toil and trouble (IV.i.10) and seemingly give him more information when he demands it - almost as if he is in control. In the same scene, he hears the ultimate when the sisters tell him that "none of woman born shall harm" and continuing the theme "Macbeth shall never vanquished be until/ Great Birnam...shall come against him. "
Macbeth believes these prophesies as they are too ridiculous to be normal and are beyond reality. What else can they be except the truth?