This is perhaps the fundamental question at the heart of Macbeth, and it is one that Shakespeare leaves unresolved. On the one hand, there is no doubt that the witches' influence helps to lead the Macbeths to their destruction, by first piquing Macbeth's ambition and then leading him to believe that he is indestructible. Some might also argue that the witches use their black arts to describe the future, doing so in such a way as to prod Macbeth into the destructive decisions he makes. In other words, they manipulate Macbeth, not events themselves.
However, it should be noted that in the final analysis, the decisions that Macbeth makes, with the encouragement of his wife in the first two acts, are his (or theirs) alone. Macbeth himself acknowledges his own "vaulting ambiton," or his "black desires" early in the play. In Shakespeare's day, the desire to usurp the crown was an unnatural one, violating the divinely-ordained order of things. Macbeth's ambition makes him susceptible to the witches' meddling, and ultimately, he allows it to consume everything that is human within him. So it is the argument here that ambition, not supernatural and malignant design, that brings about the Macbeths' downfall. Perhaps the best way to put it is that the two forces combine to bring about devastating and tragic effects.