How is Shakespeare interested in the essentials of human nature in the play Macbeth?
Shakespeare seems interested in what it would take to turn a good person into a bad one, how ambition and pride can corrupt an otherwise loyal and noble nature. There is no question that Macbeth begins the play as a good man: he is loyal to his friend, kinsman, and king, Duncan; he is willing to lay his life on the line to protect Scotland; his wife, arguably the person who knows him best, feels that he is "full o' th' milk of human kindness" (1.5.17). In his exploration of human nature, Shakespeare shows that it wouldn't take much to begin to corrupt even a man such as this: almost as soon as the Weird Sisters tell Macbeth that he will be king, he begins to dream of what it would be like. Then, when Duncan names his son, Malcolm, his heir to the throne, Macbeth immediately contemplates Duncan's murder, saying,
Stars, hide your fires;
Let not light see my black and deep desires.
The eye wink at the hand, yet let that be
Which the eye fears, when it is done, to see. (1.4.57-60)
In other words, he asks the stars to go dark so that no one will be able to see what terrible thoughts he's thinking; his eye, he says, will be afraid to see what his hand is going to do, but his hand is still going to do it anyway.
Though Macbeth admits to having "Vaulting ambition," it is evidently not enough to compel him to follow through with the murder because he tells Lady Macbeth, "We will proceed no further in this business" (1.7.27, 1.7.34). At this point, she wounds his pride, calling him a "coward" and implying that he will not be a man if he does not keep this promise to her to kill the king (1.7.47). It is only after she berates and degrades him, insulting his masculinity and pride, that he finally resolves to commit regicide.
In the end, Shakespeare seems to conclude that it doesn't take much to corrupt a good person if one can appeal to his ambition (as the Weird Sisters do) and pride (as his wife does). Then, once a person has made the initial decision to do wrong, it will take less and less to convince him to do more wrong. Human nature is revealed to be fairly corruptible, and so we must be on our guard against attempts to manipulate us.