Macbeth describes life in a fairly modernist manner in the lines after he learns of his wife's death. The initial statement that the future "creeps" in a pace defined as "petty" helps to configure human freedom as acts of futility. What is coveted and desired, what is acted upon, and what is sought above all seems to be meaningless when seen in the context of life being a "brief candle." At this point, the images of futility seem to emerge in full splendor, if this term could be employed. The notion of life being nothing more than a "shadow" helps to convey the idea of shape without form, almost as if building a castle on a firmament of sand with the time coming in to bring inevitable damage. The "poor player" helps to bring to light the idea of a bad actor, full of pomp and circumstance which is nothing more than overacting and that it in a bad manner. Again, the image here is more of freedom and its futility. The closing lines of "sound and fury" and "signifying nothing" is the capstone to a statement of life which seemed to represent so much at one point but is met with the collision of nothingness and a sense of vast emptiness. The reality of the desert was temporarily interrupted by the mirage of the oasis, but in the end, reality always emerges.
In Shakespeare's Macbeth, Macbeth despairs after he is told his wife is dead. He says:
She should have died hereafter;
There would have been a time for such a word.
She would have died sometime anyway, but she should have died at a better time than this. Macbeth's despair leads him to conclude that "...all our yesterdays have lighted fools/The way to dusty death." Our past just leads us to death, nothing more. Life is but a:
"...walking shadow, a poor player [actor]
That struts and frets his hour upon the stage
And then is heard no more."
The metaphor compares life to an actor's short career in the spotlight, after which the actor is never heard from again. Macbeth concludes with another metaphor for life:
...It is a tale
Told by an idiot, full of sound and fury,
Life is tumultuous, but in the end means nothing.
In addition to what has been explicated, Macbeth, who has already rued his murder of King Duncan--"I am afraid to think what I have done" (II,ii,49)--is chagrined by the untimely death of his wife, saying that had she died
hereafter;/There would have been a time for such a word. (V,vii,19)
That is, there would have been a better situation for his having received the message of his wife's death. But, with her untimely death, Macbeth senses the foolish attempts of man to thrawt Fate. For, man is merely
a poor player that struts and frets his hour upon the stage/And then is heard no more. (V,vii,24-25)
Now that his wife is gone, who has pushed him in his murderous path [she mocks him, "Art thou so green and pale,""Infirm of purpose!" and wearing "a heart so white."], Macbeth questions the importance and value of what he has done. Without Lady Macbeth to share his rise, his deeds seem rather pointless. And, for what purpose has she shared in his crimes if only to die? A powerful passage, Macbeth's soliloquy is, indeed, an existential reflection worthy of Shakespeare's best thinker and melancholic, Hamlet.
I believe that Shakespeare often expressed his own thoughts and feelings through his characters and that it isn't really Macbeth saying that life is but a walking shadow but Shakespeare himself. He expresses similar sentiments through Hamlet and also notably in a long speech by Duke Vincentio in Measure for Measure, in which the Duke says that it's much better to be dead than alive. Shakespeare also expresses very similar ideas throughout King Lear, where at one point the King says that when we are first born we weep that we have come to this great stage of fools. (This is a nice metaphor because all babies cry when they are first born.) Duke Vincentio says that our own children, who are really parts of our own selves, do curse the gout, serpigo and rheum for not killing us off sooner. Do we think that all these characters are real people making up their own dialogue? I believe it is a mistake to attribute Macbeth's nihilism and cynicism to the character called Macbeth based on the fact that he has just heard that his wife has died. Shakespeare is expressing his own opinions about life--that it is meaningless, a great stage of fools, a tale told by an idiot--or "vanity of vanities (Ecclesiastes)." If we attribute these thoughts to a mere character in a play they lose their universal applicability and significance. We're saying, in effect, "Oh, he's just feeling bad because his wife died and he's losing the war. We all know that life is lots of fun and full of great expectations."
This happens in Act V, Scene 5. In this scene, when Macbeth learns his wife is dead, he gives one of the more famous speeches from this play (and from all of Shakespeare). The general idea of his speech is that life is pointless.
He says that she would have died sometime anyway because everyone does. He says that life is really short, a "brief candle" and every day does nothing but bring us closer to death. He says that people act like they're all important, but their lives are meaningless (they're just illusions, like a play), "full of sound and fury, signifying nothing."
So, to Macbeth, life is short and pointless.