You must keep in mind that Shakespeare constructed Macbeth as a tragedy - complete with a tragic hero. Shakespearean tragic heroes have the potential for greatness, but through the workings of fate (weird sisters/witches) and a tragic flaw (unbridled ambition) they lose everything. While Aristotelian tragedy generally had both fate and fatal flaws working somewhat equally toward a tragic hero's downfall, Shakespeare relied on the fatal flaw to do most of the damage. Elizabethan audiences (and Renaissance audiences) were more interested in the notion of a fatal flaw and its ability to undercut even the most noble characters than they were about fate. Macbeth entertains the audience by appealing to their sense of pity for, and fear of, Macbeth the man. Macbeth is a prime example of a tragedy in which the tragic hero has the world (a perfectly honorable and respectable life) at his fingertips, but trips over his own feet and pursues a villainous route to secure the fate he thinks he has coming to him.
A couple of reasons came to mind as I read your question. First of all, the witches are an enormous influence on Macbeth's decision to pursue an evil, dark path toward murder. Had they kept their mouths shut and not done their prophecies about him becoming king, he MIGHT have remained a loyal subject to King Duncan (I say MIGHT because we really don't know...he seems to be so easily influenced by outside forces that something else might have sent him down the road to treason at a later date).
A second reason is Lady Macbeth, his wife. She could very easily have said, "Now, now, dear, let's just wait and see what happens. Maybe the witches mean that Duncan is going to name you heir, so we'll just wait and see what happens." But no, she all but pushes him into the bedroom to murder Duncan in his sleep, mocking Macbeth whenever he shows any weakness at all.
Interestingly, it's the women in his life - three witches and his wife - who set Macbeth on the path from heroism and loyalty to treason and villainy.
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