Macbeth's Witches: How foul are they really?

The Witches in Shakespeare's tragedy Macbeth have fascinated people for generations. Those of you who know the play, will recall that the action begins with the three Witches in 'a desert place'. They're talking about Macbeth, who is a Captain in the Scottish King Duncan's army. Directly after, Duncan appears on the battlefield, hears of Macbeth's bravery in suppressing a rebellion, and appoints him Thane of Cawdor. The Witches then meet Macbeth and his fellow Captain Banquo when they're returning from battle and announce that Macbeth's Thane of Cawdor and that he will be King. Banquo's children will be king but he won't. The evil sisters' announcement set the ball rolling for Duncan and Banquo's death and Macbeth's elevation as King. But just how responsible are the Witches for the tragedy that plays out? That's a question every Shakespeare scholar mulls over. So brilliant is the Bard's genius that we're never really sure...

Obviously Macbeth already has evil in his heart. For no sooner does he learn of his appointment as Thane of Cawdor than he begins to mumble: 'why do I yield to that suggestion/Whose horrid image doth unfix my hair/
And make my seated heart knock at my ribs/Against the use of nature?' He goes on and on about such evil thoughts, ignoring Banquo's warning:' 'tis strange:And oftentimes, to win us to our harm/The instruments of darkness tell us truths,/Win us with honest trifles, to betray's/In deepest consequence.' 

I'm focussing on this as a study of an efficient narrative tool: ambiguity. Wouldn't it be great if one could come up with such characters that take the action forward and leave us thinking about cause and effect for a long, long time after the story's over?