I would say it is the section beginning with “Tomorrow, tomorrow and tomorrow.” This is when Macbeth learns of his wife’s suicide. He goes from evil ambition to guilt and here, to complete nihilism. He views life as meaningless, noting that all his past (yesterday) have simply led (lighted) the way to the inevitable: death. He compares (his) life to a fiction, a play, where he the character can scream and make sound and fury on the stage. The play will end. Like a play, Macbeth’s life was scripted (witches’ prophecies); since he believes he has no control over the events, they are meaningless to him. But Macbeth has chosen nihilism and thus accepts his death almost with open arms. Remember that it is unclear whether the witches claim that Macbeth’s future is fate; or if they just have foreseen what path Macbeth will choose. I think it is the latter. Macbeth chose the path. The witches were forecasters. In this speech, Macbeth does not understand this. He thinks he is just a pawn in some ridiculous, meaningless play; told by an idiot.
While it is true that Shakespeare did not give Macbeth a death speech, in Act V, scene 5, he ponders life and death when he is told by Seyton of Lady Macbeth's death. His response is the famous "Tomorrow" speech.
If you take the speech and look at it line by line, you can see that Macbeth is looking at time itself. He looks at the past, present and future. Even the references he use , IE the candle, is a measure of time. In fact, the last word of each line becomes a summery of what he discovers.
He looks over time and realizes that in the end, we all come to the same end. As he says, life is "a tale told by an idiot, full of sound and fury, signifying nothing.