What is the theme of Shakespeare's Macbeth?

Macbeth gives us the main theme when he says in Act 1, sc. 7, "I have no spur / To prick the sides of my intent, but only / Vaulting ambition...".  The witches know that Macbeth is an ambitious man and they take advantage of that ambition by nurturing that seed.  They tell him, in sc. 3 of the first act, that he will become Thane of Cawdor and King.  When he, almost immediately after their prophecy, does become Thane of Cawdor, then that ambition is fertilized and allowed to grow and blossom into the full-fledged monster it becomes for Macbeth.  Macbeth's ambition blinds him to all other things. He has his own friend, Banquo, killed because he fears that Banquo might suspect he killed Duncan.  Macbeth's ambition makes Macbeth become so further paranoid that he goes back to the witches for more prophecies.  When one of the prophecies says to beware of Macduff, Macbeth seals the fate of Macduff's family even though the next two prophecies seem to make Macbeth's reign secure.  Nonetheless, Macbeth has Macduff's family slaughtered since Macduff himself, is out of the country.  Even in Act 5, when his world is disintegrating around him as his wife dies, Malcolm along with Macduff and the English, are advancing on his castle and most of Macbeth's men leave him, he still fights to hang on to his throne.  Blinding ambition is what the play is about and what happens when one allows his ambition to overrule all other senses.