Shakespeare used the witches in the play to give the audience a sense that the witches are in control and very manipulative as they lured Macbeth to perform evil deeds. Shakespeare might have wanted to bring a little excitement and drama into the play. At the very opening of the play they create a sense of mystery by saying: ‘Fair is foul and foul is fair’ which constitutes the main action of the play.
Macbeth is fully capable of doing all the mischief and evil on his own. At the start of the play, Macbeth and Banquo are returning from the battlefield when they meet the witches. At this time they predict that Macbeth will become Thane of Cawdor and king of Scotland. It is an interesting thought and the start of an idea. He has fought bravely for king and country, but when the first prophecy comes true, and he is made Thane of Cawdor , he says to himself, “ The first step toward the ultimate goal, the throne.” If he calls the ultimate goal, a throne, then he has been entertaining this idea before.
In his life he has prioritized his ambitions, and the title of king is what he considers the highest step. Ambition drives Macbeth. He only needs the suggestions of things that might be his to push him on. There is no sense of moral right to keep him from murder. He hesitates only because he fears the earthly consequences not because it would be sinful. “If the assassination could trummel up the consequences.”
The witches do not command Macbeth to kill Duncan or anyone else. The murder of his king is his decision. This is the only way that he can see to reach his “Ultimate Goal”. One murder leads to another. Macbeth has spun a web that has trapped him in a paranoid mess. Soon he believes that everyone is out to get him. Traitors are behind every stone in his castle. He has no trusted friends left, and even his wife has fallen into a pit of madness. The only way to deal with this is to kill and kill again. He must know what the future holds for him, and again he turns to the witches. May be they can reassure him.
At this stage of the play, Macbeth is in desperate need of some measure of security. The witches are only too happy to oblige. They’ll give him just what he wants-- almost. Hecate has forecasted Macbeth’s weakness when she tells the three witches. Now they tell him to beware Macduff, that no man born of woman will harm him, and that he will not be beaten until Birnum Woods marches on his castle. Macbeth is reassured. How can a man not be born of woman, and woods do not walk. He has heard just what he wanted to hear.
The last two warnings are what he thinks about and he all but dismisses the first: “Macbeth, Macbeth, Macbeth! Beware Macduff. Beware the Thane of Fife!” Had he listened carefully to the first warning, he undoubtedly would have found a way to kill Macduff. But, again the witches have given Macbeth what for he thought he needed. Time and again, the witches appear in the play. They warn, predict and tempt, but they do not control Macbeth. He is the master of his own fate. He controls his own life. The decisions are his as well as the sins of his deeds.