In Macbeth, the presence of the witches provides the main character, Macbeth, with incentive, or motivation, which awakens in him, his deeply held desire to be king. The witches are not responsible for Macbeth's downfall, he chooses to kill the king because he suffers from unchecked ambition, which is at the heart of his decision.
However, I would agree that Lady Macbeth had more to do with Macbeth's decision than the witches. She becomes inspired with the idea to kill the king, once she receives the news from her husband about his elevation to Thane of Cawdor.
The witches conspire to influence Macbeth, in Act I, they make a decision.
"First Witch. Where the place?
Sec. Witch. Upon the heath.
Third Witch. There to meet with Macbeth.
First Witch. I come, Graymalkin!
Sec. Witch. Paddock calls.
Third Witch. Anon.
All. Fair is foul, and foul is fair:
Hover through the fog and filthy air. [Exeunt." (Shakespeare)
After Banquo and Macbeth encounter the witches and the prophecy is given to both of them, Macbeth does not really believe what the three witches have told him. But then he receives news the shocking news that he will be made Thane of Cawdor, just as the witches said.
"Macb. [Aside.] If chance will have me king, why,
chance may crown me,
Without my stir.
Ban. New honours come upon him,
Like our strange garments, cleave not to their
But with the aid of use.
Macb. [Aside.] Come what come may,
Time and the hour runs through the roughest day.
Ban. Worthy Macbeth, we stay upon your
In this exhange, Banquo and Macbeth discuss the fact that the prophecy, at least part of it has come true. Macbeth states that if he is to be king, it would happen without him doing anything at all. Banquo believes that Macbeth is receiving the benefit of hard work, that he is worthy. So at this point, Macbeth does not see any reason to kill the king.
Later in the play, when Macbeth tells his wife of his new honor and that the King is coming to their home for dinner, she can't believe her good fortune. It is her idea to kill the king.
"Lady M. Give him tending;
He brings great news—[Exit Messenger.] The
raven himself is hoarse
That croaks the fatal entrance of Duncan
Under my battlements. Come, you spirits
That tend on mortal thoughts! unsex me here,
And fill me from the crown to the toe top full
Of direst cruelty; make thick my blood,
Stop up the access and passage to remorse," (Shakespeare)
In this speech, Lady Macbeth longs to be made a man so that she can carry out the deed of killing the king. She comes up with the plot to murder Duncan, the only reason she does not do the killing herself is because when she enters the king's bed chamber, she says that the sleeping king looks too much like her father.
Macbeth is influenced more by his wife than the witches, it is Lady Macbeth who, along with Macbeth, himself, who bear the most responsibility for the death of the king and Macbeth's ultimate downfall.
"Some critics maintain that responsibility for the deaths of Duncan and Banquo rests solely with Macbeth, whose own ambition and nature are the cause of his deeds."
"Others cite Macbeth's reluctance prior to Duncan's murder and argue that Lady Macbeth goads her husband into the action. Lady Macbeth does, however, set the time and the place of Duncan's murder, claims that she would kill a baby at her breast to honor a vow, and argues that when Macbeth first conceived of killing Duncan, then he was a man."