What other roles do the witches play in Macbeth's downfall (in addition to how they entranced and manipulated him by prophesying him becoming King)?
One other important element of the witches' role in Macbeth by William Shakespeare is the way they serve as a moral test. In Shakespeare's era, Christianity was the official state religion in Britain, and the play Macbeth is set in Scotland during a period when everyone would have been a member of the Roman Catholic Church. In this environment, witches would have been considered agents of the Devil, and so evil that even talking to them might imperil one's immortal soul.
Although Banquo, after initially hearing the women, realizes that they are evil, Macbeth is will to risk his soul for the important information the women may provide. What this does is suggest to the audience quite early in the play that Macbeth, despite his initial heroism is morally bad. Thus their role is not so much causing his downfall as revealing and enabling the evil elements and ambitions already present in his character.