in my point of view the tragic flaw in macbeth is being over trusting because he trusts the witches who lead him to his ultimate downfall BUT i'm convinced in the idea that ambition is the tragic flaw ..... so what do you think???
You might also argue that Macbeth was over trusting of his wife. For example, he allowed himself to believe that he would not suffer a guilty conscience after Duncan's murder. Lady Macbeth consoled him by stating "A little water clears us of this deed" (Act 2 Scene 2). Allowing himself to be manipulated by her in murder was part of his character flaw.
There isn't much to say that he actually trusted the witches. He only had negative comments to make about them, and really only saw them as sources of useful information which drove his lust for power. As Macduff moves in to finish his life Macbeth realizes too late that their prophecies were in fact only loaded with deceit in order to cause chaos and spread evil.
Macbeth himself says that it is his "vaulting ambition" that urges him to help along the predictions of the three sisters. After he becomes king because of Duncan's death, Macbeth is not satisfied with his position, paranoiac that Banquo or Macduff will take it from him. His tremedous desire for power constantly pushes Macbeth.
Macbeth is both too ambitious and too trusting, but I would say that neither is his flaw exactly. His flaw is that he is too easily manipulated. The witches and his wife about able to make him dance like a puppet.
I would argue that it is Macbeth's ambition that causes him to put so much faith in the witches' prophecies, or at least not to question their meanings more. As soon as he hears their prediction that he is to be king, he (and, perhaps just as significant, his wife) is consumed by "black desires." So at the end of the day, if you accept that Macbeth acts out of free will, which, as some of the contributors on enotes have correctly pointed out in recent days, is highly debatable, then I think you have to say it is his ambition that drives him to the evil deeds he commits.
Macbeth's ambition leads to his downfall. Sure, he naively trusts the witches' prophecies, even against Banquo's wise assurance that their purpose might be evil, but his ambition is what leads him to take action--to murder Duncan, to murder Banquo; Macbeth becomes obsessed with power and the throne.
If we see the witches as being factors of reinforcement for Macbeth's ambition - instead of the creators or generators of that ambition - we can say that Macbeth's ambition is what led to his downfall.
The witches do lead Macbeth to have more confidence that he would have had without them, but the will to power was already in him.