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.