The supernatural elements in Macbeth add an intriguing dimension to the play and help to establish and intensify the general atmosphere of darkness and evil. As to actual events, however, it seems that the power of the three weird sisters - the chief representation of the supernatural - lies in the art of temptation rather than in direct action. They have a fateful influence on Macbeth, playing on his ambitions and tempting him to sin, and also misleading him with their equivocal statements.
However, Macbeth is not merely the plaything of supernatural elements, of external forces. He chooses to believe the witches' prophecies because it suits his ambitions to do so, and in the end it is his own choice to murder Duncan. Indeed, it is not the witches but his own wife who proves to be the critical influence in this murder. At one point he declares: 'We will proceed no further in this business' (I.7.31) only to unleash a tide of mockery from Lady Macbeth:
When you durst do it, then you were a man;
And to be more than what you were, you would
Be so much more the man.(1.7. 49-51)
Lady Macbeth follows this up with her infamous assertion that she would readily tear her baby from her breast and dash its brains out if doing so would prove the strength of her resolutions (1.7.54-59). Such a horrific image proceeds not from supernatural beings but from the well-do-to wife of an honoured general.
Essentially, the supernatural realm in Macbeth serves to bring out and to highlight the capacity for evil that already exists in human beings. It gives a lurid shape and form to the human potential for evil, but it does not cause it; it is Macbeth's own ambition and subsequent sense of guilt that ultimately leads to his destruction.