First, we should acknowledge that there are certainly elements of the supernatural, or fate, at work in the play. Macbeth is certainly influenced by the witches' prophecies, and of course they help lead him to his destruction. Others point out that his wife plays a major role. That said, it can certainly be argued that Macbeth is led to destruction by his own free will. Macbeth's ambition for the crown makes him susceptible to the witches, and willing to commit horrible crimes, as he himself admits just before killing Duncan:
I have no spur
To prick the sides of my intent, but only
Vaulting ambition, which o'erleaps itself
And falls on the other...
The witches never force Macbeth to do anything. They predict that he will be king of Scotland, but it is Macbeth who assumes the prophecy means he must kill Duncan (and Malcolm, which he is unable to do.) They show him and Banquo the vision of a line of kings descended from Banquo, but Macbeth determines from this that he must murder his friend. They warn him through visions to "beware the Thane of Fife," but it is Macbeth who decides to have Macduff's family murdered. Many might argue that the witches are responsible for providing him with the visions to begin with, but they never tell him how to act on them. He does this of his own free will.