Well, Macbeth is certainly no simplistic story, so I can see why it might be said it has no single meaning. I would tend to say, however, that its primary theme is the corrupting results of ambition. Its been a very long time since I read it, all the way back in grade 11 four years ago, but I seem to remember my techer pointing out especial his comment on his "overvauting ambition" or something to that effect. Macbeth's ambition goes too far, and I think he realizes this himself by the end. He's listened to three witches, killed his king dishonorably, killed his best friend, and, in so doing gained power. But ever bit of power he gains, the more evil he becomes, and makes enemies.
