From the moment the weird sisters tell Macbeth and Banquo their prophecies, both the characters and the audience are forced to wonder about fate. Is it real? Is action necessary to make it come to pass, or will the prophecy come true no matter what one does? Different characters answer these questions in different ways at different times, and the final answers are ambiguous—as fate always is.
Unlike Banquo, Macbeth acts: he kills Duncan. Macbeth tries to master fate, to make fate conform to exactly what he wants. But, of course, fate doesn't work that way. By trying to master fate once, Macbeth puts himself in the position of having to master fate always. At every instant, he has to struggle against those parts of the witches' prophecies that don't favor him. Ultimately, Macbeth becomes so obsessed with his fate that he becomes delusional: he becomes unable to see the half-truths behind the witches' prophecies. By trying to master fate, he brings himself to ruin.