The Power of the Witches in Shakespeare's Macbeth
The tragedy of Macbeth comes about because of a single event in his life. If that one moment, the meeting with the witches on the heath, had not happened then Macbeth would no doubt have gone on to be a loyal and respected subject of King Duncan and, later, King Malcolm. However, the meeting did happen and the powerful force of ambition was unleashed within Macbeth and Lady Macbeth. It is the combination of these two factors, the meeting with the witches and Macbeth's own inner demons, that lead to tragedy, and make the play 'terrifying' in the Aristotelian sense.
The three witches are certainly responsible for initiating the events that lead to Macbeth's tragedy. Their greeting to him
All hail, Macbeth! Hail to thee thane of Glamis!
All hail, Macbeth! Hail to thee, thane of Cawdor!
All hail Macbeth! That shalt be king hereafter!
feed straight into his desire for advancement. At this point in the play he is the newly appointed thane of Glamis but assumes that the thane of Cawdor still lives. When news arrives that he is to be the new thane of Cawdor, Macbeth sees the second greeting as a prediction and cannot help but wonder whether the third greeting will also prove accurate. Banquo says that the forces of darkness use the truth to win us to harm but Macbeth is unsure.
This supernatural soliciting cannot be good,
Cannot be ill.
For the audience there is even more to think about. They know from the conversation about the sailor whose wife had offended one of them that the witches' power is circumscribed.
They can torment him but not change his fate.
Though his bark cannot be lost
Yet it shall be tempest tossed
The audience are also aware that the second greeting is not a prediction as the order to greet Macbeth with the title 'Thane of Cawdor' was given by Duncan in the previous scene. There can be little doubt that the witches are exploiting the situation for their own evil ends and are using "honest trifles" to win Macbeth to harm, but given the limited nature of their powers, it is hard to say that they are responsible for Macbeth's later actions.
Like the witches, Lady Macbeth is crucial to the actual accomplishment of Macbeth's crime. Without her, Macbeth would not have carried out the murder in the first place - "we shall go no further in this business" - and without her timely interventions in gilding the groom's faces with blood and conveniently fainting when Macduff's questions become too insistent, it is unlikely that he would have got away with it. She seems to be just as ambitious as her husband and the plan to kill Duncan is largely hers. She overcomes Macbeth's scruples by both encouragement and scorn:
Macbeth Pr'ythee, peace!
I dare do all that may become a man;
Who dares do more is none.
Lady Macbeth What beast was't, then,