The three witches in the tragedy are introduced right at the beginning of the play and the brief opening scenes instantly give an impression of mystery, horror and uncertainty. This is a sort of omen of things to come, with the witches creating an atmosphere of evil and disorder.
The seeds of evil in MacBeth are not planted by the witches, though; they are only cultivated by them. His wicked ambition is already at work before his first encounter with them, this ambition made more prominent by the contrast in reaction between himself and Banquo.
Banquo is deeply distrustful of the hags, and believes that these prophecies will only bring harm even before anything begins to happen. He sees through the witches’ veil of riddles, recognising their premonitions of doom. For example, Banquo says in act one scene 3 lines 124-125:
"The instruments of darkness tell us truths, / Win us with honest trifles, to betray's/ In deepest consequence."
MacBeth on the other hand at least convinces himself that the success they seem to guarantee will be without the small print. If MacBeth had listened to Banquo then perhaps the tragic events to follow could have been stopped.
Their very foreknowledge of their meeting with MacBeth implies they see a flaw in him, which they would manipulate.
In the opening scene, the weather is thunder and lightning which is a mirror image of the way the witches are perceived. Evil and destruction are associated with stormy weather and this is exactly the way witches are represented in this play.
The witches are indeed maintained as the centrepiece of the play, because it is they that offer the protagonist the real temptation to commit murder. If Thomas Cooper is correct in his The Mystery of Witchcraft, then “Satan cannot preuail effectually upon any of their condemnation, unless with full consent they yield themselves…”. In this case the guarantee of success and security that they seem to offer him proves too good to ignore, though this does not necessarily imply evil means of achieving this prophesy of the crown. Even after Duncan’s murder, he does not think to use the witches as scapegoats for his crime, knowing that it was completely his own responsibility, admiting that:
Chance may crown me
Without my stir
The witches only proclaim the prophesy to MacBeth as they realise the extent of his ambition, and the contrast in the way that he and Banquo receive this information acts as further testimony of MacBeth’s flawed character, succumbing to his pride.
The idea that the witches are central to the play could be taken further if one accepts that Lady MacBeth, though hardly a “secret, black and midnight hag”, could be considered as a witch by the standards of Shakespeare’s time. Lady MacBeth is really, at least until the first murder, in the driving seat of the relationship, demonstrated in her soliloquy, willing MacBeth to "Hie thee hither, /That I may pour my spirits in thine ear". This subversion of gender is evident in her actual influence over the thoughts and actions of her husband. In the preface of King James’ Daemonologie, he claims that witches have power for weaken “the nature of some men, to make them unavailable to women”. This, it would appear, is exactly what she does, by appealing to the ‘fragile male ego’- striking at his very manliness, taunting his masculinity, claiming, “When you durst do it, then you were a man”[I:VII,48]. If Lady MacBeth was not aware of the witches herself, then she certainly is indirectly allied with them, spurring on MacBeth’s latent ambition with her own ambition and wicked femininity.
Furthermore, seemingly malevolent spirits are called upon in Act 1, Scene 5, in which she invokes them: “Come, you Spirits, /That tend on mortal thoughts, unsex me here…”. This sinister summons smacks of a deliberate alignment with evil, to stifle her conscience- a direct contrast to MacBeth’s agonies. She willingly gives herself over to these powers, as Mrs Simmons declared that Lady MacBeth, “having impiously delivered herself up to the excitements of hell…is abandoned to the guidance of the demons she has evoked”.
The witches can also be compared with the Freudian concept of the repressed conscious, becoming symbols of MacBeth’s ‘primary urges’, and essentially chaotic subconscious. This would explain succinctly why Lady MacBeth’s appeals have such a powerful effect on him. The witches would actually be the heart of the play if they were thought of as representing his deeper emotions, though ones only allowed to surface occasionally, in a kind of dream. In fact, the witches, mutatis mutandis, could be dream-like apparitions. Like dreams, they do not keep to spatial reality or time, they show some connection to real life because the witches seem to show what MacBeth desires are and how he can get them. This is very similar to a dream- as suggested by the phrase ‘a dream come true’- which might show what we want and desire. Both dreams and the witches are a blurred form of reality; neither do they conform to any fixed structure. The witches are rebellious by nature, suggested by (amongst other things) their ‘chorus’ “Double, double, boil and trouble”, and by raising the thought of rebellion in MacBeth, lead him to his eventual downfall and death. This definitive triumph of good seems intrinsically linked with the paramount message of the play, displaying how evil thoughts only need a little weakness of will to turn into actions. This Christian moral is deeply embedded in the play, as shown by MacBeth receiving the ultimate retribution for allowing the evil of ambition, then of power to corrupt him utterly.
Thus, the conclusion that one can reach is that MacBeth was not just a hopeless victim of fate. He may have been guided by evil influences, but in the end, he made the choice to kill Duncan, the guilt following which led to his insecurity leading to his further murders. Therefore, though fate may have played a part in his life he made his choice and had to face its consequences, showing that we can’t just sit back and blame "fate".