Macbeth starts the play as a ferocious and popular warrior, awarded honours by his king, and loved by his men. He ends it, in the words of Malcolm, as a "dead butcher" with a "fiend-like queen". What happened in between? He killed the reigning monarch, usurped his throne, and then, to secure the throne, murdered a whole host of others.
The question is - why? Why does Macbeth go on the traditional tragic trajectory from glory and good fortune all the way down to dishonour and death?
I'd argue that there are three main options:
1) His own ambition. He admits that he has "black and deep desires", very early in the play, and his response to the prophecy that he will be King ("...to be King stands not within the prospect of belief") suggest very strongly that he has already considered the crown. You also have to think that he wouldn't murder Duncan unless - somewhere - he wanted to. Lady M may be his "dearest partner of greatness", but he still has a half share.
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2) The ambition of his wife. Lady Macbeth decides as soon as she gets his letter that she will remove "all that impedes thee from the golden round". She determines to get him the crown, and, when he backs out of the conspiracy to murder Duncan, she blackmails him - insulting his manhood, and mentioning their dead child - in order to get him back on board. Yet, after the Macbeths are crowned, her influence decreases hugely: Macbeth dismisses her, she goes mad, and ends up committing suicide.
3) The witches and the supernatural. The play begins with the witches, and they do cast some sort of spell shortly before Macbeth's first entrance ("Peace! The charm's wound up!). A dagger seems to appear to Macbeth before Duncan's murder to encourage him, though it "marshallest" him "the way that I was going" anyway. Is Macbeth the victim of some sort of supernatural spell - is he not in control? This argument makes the play, arguably, much less of a tragedy, as it reduces Macbeth to a powerless puppet. But there is certainly a strong supernatural element running underneath the play.