What does Macbeth convey about humanity or the human experience?
Much as in Robert Frost's poem "The Road Not Taken" where "way leads to way, Macbeth and Lady Macbeth's murderous path in which "blood will have blood" leads to their destruction. For ambition--"vaulting ambition"--left unchecked destroys its owner.
When Macbeth forces the predictions of the three sisters by killing King Duncan, then he enters a realm of blood, paranoia, insomnia, and phantasmagoria. Clearly, the "evil that men do" affects not only their victims, but themselves. For, the Macbeths continue their evil paths until they destroy themselves. The guilt of the murders of King Duncan and the two guards, along with the heinous deeds of her husband, lead Lady Macbeth into madness. And, in his madness, Macbeth commits preemptive murders, slaying Banquo, then attempting to kill Fleance; he attempts to destroy Macduff, but kills his wife and son instead. So immersed in blood and the phanstasmagoric realm is Macbeth that reality and fantasy become equal to him: "nothing is what is not."
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,
Certainly, when the blood of others lies upon the hands of those of "vaulting ambition," they follow a path of murderous destiny that takes them from reality and leads them to madness.