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It is ultimately that he can beat/defeat the witches' prophecies, which ironically led him to that point anyway. He tries to beat Banquo's future lineage by having Banquo and Fleance killed. He is, of course, unsuccessful because Fleance survives allowing the prophecy to be fulfilled. He tries to defeat the prophecy of "Beware Macduff" by killing Macduff's family. He is initially unconcerned with the final two prophecies because he views them as impossibilities, even though everything else the witches have said have come true. His hubris and total lust for power blind him to these "truths." It also ultimately blinds him the Lady Macbeth's unraveling. He was originally the one with concerns and misgivings, but once he had the taste of power, there was no going back. Lady Macbeth, however, cannot reconcile herself to what they have done, but Macbeth's hubris does not allow him to help her or even see that she needed help until it was too late.