Macbeth is shocked and talks about how life is "a tale told by an idiot, full of sound and fury, signifying nothing".
Macbeth realises that he has lost everything now, including the wife he had loved so much. This shows how the loss of his wife, and his increased power, have completely changed Macbeth. He feels there is no point to life anymore. But this attitude about life having no meaning could also suggest he is trying to justify his crimes – is he suggesting that the murders he has committed are less horrible because life, and death, are meaningless?