The witches are merely in instrument in Macbeth's destruction. The mechanics of it are entirely of his own invention. Their role in this boils down to the seductive power of suggestion.
Macbeth organizes the most nefarious plots which become the key to his undoing:
- Duncan's murder. Macbeth wishes that "th'assassination could trammel up the consequence" (1.7.2-3). He wants all of the benefits that would come from killing the king, without shouldering any of the responsibility.
- The hiring of the murderers to kill Banquo and Fleance. He feared the witches' prediction that Banquo "should be the root and father of many kings" (3.1.5-6)
Ultimately Macbeth's own greed and ambition for the crown lead him to murder and deceit. He could have chosen a completely different route upon hearing the witches' dark prophecy.
Yes, you are right, they merely lead him. Actually, when ever a man have any ambition (desperate/ and not healthy) he can't differentiate what is right and what is wrong?
Things occure and he only accepts those which is suited to his ambition without seeing the dark or seemy side of the action.
Witches are only the means, this is the demarit of Mecbeth that he couldn't control himself and that is why he was so sad and behaved like mad person after murdering Duncan.