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.