What role does fate play in Macbeth?

This is actually a question that has been much debated. Do the Weird Sisters actually have knowledge of the future because there is such a thing as fate?  Or, are they simply telling Macbeth something that might ignite his ambition because they want to see what he's capable of doing to make it happen? Unfortunately, it is never completely clear.

Many argue that fate is not involved in Macbeth's tragedy, that it is the result of his own free will and poor decisions.  Therefore, when the Weird Sisters tell Macbeth that he will be Thane of Cawdor, this actually isn't a prophecy because it has already happened.  Macbeth just doesn't know that Duncan has given him this new title, and so it seems to him that the sisters can tell the future.  Then, since they've also told him that he will be king, he assumes it is true because their first statement came true.  However, it could be that Banquo is right when he says,

"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" (1.3.135-138). 

What he means is that the sisters might have told Macbeth one small truth in order to convince him to believe an even bigger claim, a claim that will result in his ultimate betrayal.  We know, too, that the Weird Sisters are vindictive and malicious and seem to get a great deal of enjoyment from messing with people (think of the poor man whose wife wouldn't share her chestnuts).  Perhaps they simply tell Macbeth that he'll be king in order to manipulate him into corrupting himself, a process which they can watch and by which they can be entertained. These are all reasons to think that fate does not play a role.

On the other hand, it is clear that the Weird Sisters are supernatural.  They can vanish, first of all; secondly, they are in league with Hecate, the goddess of witchcraft, who can legitimately do some seriously freaky stuff (consider the apparitions she conjures).  They know that Banquo's descendants will be kings, and Banquo clearly doesn't do anything to make that happen. (James I of England, a descendant of Banquo's, is actually on the throne when Macbeth is first produced.) So, they appear to have some knowledge of the future, unless they are just guessing right (and that seems unlikely).  Also, the apparitions that Hecate produces do say true things about the future as well.  If the witches truly have knowledge of the future, then fate must rule, and the "decisions" that Macbeth makes are only those that fate allows him to make.  In this case, he is almost exonerated from blame, though, because if this is fate and he cannot help but do what fate commands, then it is difficult for us to hold him responsible for all the terrible things he does.  And it doesn't seem like we're supposed to let him off the hook (especially considering that he gets worse and worse as the play progresses).  So, that puts us back at square one: perhaps there is no fate, only free will.

So, what is the role of fate in Macbeth?  There is, frankly, evidence to support the claim that fate rules Macbeth, just as there is evidence to support the claim that the characters have free will.