What does Shakespeare's Macbeth say about the nature of man?
At the beginning of the play, Macbeth is a loyal servant to the king and a war hero. There is no indication of him having evil inclinations in any way at this point. Macbeth does not begin to consider or commit evil deeds until the prophecies/suggestions of the witches and the encouragements of his wife. After he's committed murders, Macbeth is consumed with guilt and fear of his crimes being discovered. Those things being said, he is not inherently good nor inherently evil; he is capable of both.
From Macbeth's point of view, he might feel his free will is in a battle with fate. While he might understand that humans are capable of good and evil, he surely must feel (to some extent) that he is fighting a difficult battle against fate itself. After learning that his wife has died, he is still determined to fight on but he has a moment when he considers the insignificance of his life:
Tomorrow, and tomorrow, and tomorrow
Creeps in this petty pace from day to day,
To the last syllable of recorded time; (V.v.21-23)
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,
Signifying nothing. (V.v.26-30)
If anything, Macbeth shows the duality of man (humanity). Humans are capable of good and evil. When Macbeth succumbs to the allure of power by means of evil, he develops from a loyal hero to a murdering tyrant. This shows the good/evil duality of humankind.
Since Macbeth was essentially persuaded to go down this evil path, the play also shows the temptation of evil, whether through supernatural (witches) or human (Lady Macbeth) influence. Note from the quote above how Macbeth is frustrated with battling the fates and his own feelings; he determines that life signifies nothing. This shows how, when dealing with good and evil influences in the world, people (like Macbeth) can despair to the point that they think life is meaningless. In the play, the nature of man is the good/evil duality. But man's nature is also a dual struggle between free will and the good and evil influences around us. Life only becomes meaningless when we cease trying to understand these dualities in order to make wise decisions.
In Macbeth Shakespeare presents the argument that man by nature is corruptible. Initially alarmed by the witches, Macbeth soon only wants to hear more about himself (Act I, Scene 3, line 73). The witches give Banquo a riddle with a contradictory set of qualifiers. He will be less than Macbeth, but somehow more and "get kings, though thou be none" (Act 1, Scene 3, line 70). Macbeth seizes on the ideas presented by the witches' proclamation, which appears to be a prophecy because they hail him "thane of Cawdor" and then that title is granted to him. The witches also say that Macbeth will become king, so Macbeth reasons that becoming king is also a given. Because he wants to be king, Macbeth takes action to seize the crown. Since the prophecy about himself came to pass, he interprets the riddle put before Banquo as a direct threat to himself. Macbeth readily accepts supernatural declarations that he wants or fears to be true. He becomes king through his actions, but he also becomes a murderer and a tyrant.