When the witches tell Macbeth that he will be king, in act 1, scene 3, he himself makes the point that
if chance will have me king, why, chance may crown me,
Without my stir.
It is in the following scene, when Duncan confers on Malcolm the title of prince of Cumberland, that Macbeth begins to think that he may have to act in order to seize the crown for himself and begins plotting to do so.
The witches do not tell Macbeth to murder Duncan, and even if they had done, he would not have to obey them. It is notable how much Macbeth and Banquo, both noblemen and generals from similar backgrounds, differ in their treatment of the witches. Banquo, who is modest in the company of those he considers his equals, treats the witches with haughty contempt, refusing to be influenced by anything they say. Macbeth is fascinated by the witches and quickly allows himself to become a tool for their use.
Macbeth is influenced by both the witches and his wife, but in the end, his decisions are his own. He soon shows an aptitude for tyranny and murder. The witches tell him to beware Macduff, but they do not suggest that he should kill Macduff, let alone Macduff's wife and children. Shakespeare makes it clear that Macbeth's mistake lies not in believing in the witches' power. Witchcraft appears to be real within the world of the play. His mistake lies in trusting the witches, and believing that they mean anything other than harm to him, or any other human beings. It is his lack of any moral compass of his own that makes him so eager to collaborate with the powers of darkness.