The witches' role in Macbeth's downfall is that of amused spectators who merely set events in motion, so they could watch the outcome. They speak prophetically to Macbeth, but do not force him to act or even tell him what to do.
Macbeth engineers the plots which ultimately lead to his downfall. In his soliloquy in Act I, scene 7, Macbeth compares his "vaulting ambition" to a "spur to prick the sides of my intent" (1.7.25-26). He understands that these choices are his to make and recognizes that ambition motivates him to act against Duncan.
Later in the play, Macbeth also single-handedly plans the murder of Banquo and Fleance, fearing that Banquo "should be the root and father of many kings" (3.1.5-6) This line comes from the witches' original prophecy, but Macbeth uses it to convince himself that Banquo and Fleance represent a threat to his reign as king.
Ultimately, Macbeth's ambition proves to be his true undoing. He has no one to blame but himself.