How did the supernatural contribute to Macbeth's death?

Though it's Macbeth's own ambition that ultimately causes his downfall, his demise is helped along by the supernatural forces that lurk on the fringes of the play. Indeed, it's the Weird Sisters' prophecy that convinces Macbeth to embark on his murderous rampage in the first place. Though the witches are supernatural beings who don't seem to be fully part of the normal, conventional world, they don't seem to have any problem toying with the lives of humans, as they tell Macbeth deceiving half-truths that encourage his rise to power and, later on, his false sense of security. Thus, though Macbeth makes personal choices that lead to his decline and eventual death on the battlefield, his decisions are influenced by the supernatural forces represented by the Weird Sisters. It makes one wonder what might have happened if Macbeth had never encountered the three witches.