In this scene, two events occur that arouse a sense of fatalism in Macbeth. First, he receives news that his wife has committed suicide, an event that elicits a brief speech that is astonishingly bleak:
Tomorrow, and tomorrow, and tomorrow
Creeps in this petty pace from day to day
To the last syllable of recorded time;
And all our yesterdays have lighted fools
The way to dusty death. Out, out, brief candle!
Life's but a walking shadow, a poor player
That struts and frets his hour upon the stage
And then is heard no more. It is a tale
Told by an idiot, full of sound and fury,
Life for Macbeth at this point has no deeper meaning, no purpose or significance. It is only a march toward death, and a brief one at that. No sooner has the title character completed this dreary observation than a messenger brings more bad news: Birnam Wood appears to be marching on Macbeth's stronghold at Dunsinane. Of course, these are Malcolm's troops, carrying boughs as camouflage, but Macbeth immediately (and correctly) interprets it as a fulfillment of the witches' prophecy that he could only be destroyed wihen Birnam Wood marched on the castle. Macbeth recognizes that he is cornered, and that the end is probably near, and he responds by ordering his men to attack:
I ’gin to be aweary of the sun
And wish the estate o’ the world were now undone.
Ring the alarum bell! Blow, wind! Come, wrack!
At least we'll die with harness on our back.
Driven to further nihilism, he claims to welcome his own destruction, and observes that he can at least die bravely (with his harness, or armor, on his back.) So this scene finds Macbeth responding to bad news with an almost existential view: Life is meaningless, so what does it matter if it ends now? Still, in a moment of self-deception, he holds out hope, and seems confident that he can still prevail until he discovers that Macduff was born by caesarian section, and is "not of woman born," thus fulfilling the second part of the witches' prophecy. At that point, he feels tricked by the malevolent forces of the witches, and tries to withdraw from the fight.