How do Shakespeare and Golding depict the inherent nature of humans throughout Macbeth and Lord of the Flies? Give at least three clear instances that can be developed further.

Interesting question! Both William Shakespeare’s tragedy Macbeth and William Golding’s novel Lord of the Flies offer a bleak interpretation of human nature. Let’s examine each literary work individually and provide examples of the author’s pessimistic view of human nature from both Shakespeare and Golding.
Macbeth chronicles the fall of a Scottish thane who, at the beginning of the play, is a loyal subject of King Duncan and brave warrior. Initially, Macbeth seems honorable, and others praise his valor. However, Shakespeare believes that even the purest person can be twisted to perform evil actions. By introducing Macbeth as a stellar warrior and honorable man, Shakespeare reveals his belief in the corruptibility of human nature.
In Macbeth’s case, his ambition and desire for power lead to the murder of King Duncan. But Shakespeare takes pains to emphasize that this murder does not come naturally to the Thane of Cawdor. Macbeth attempts to call off the plan in Act I, scene 7 when he says,

We will proceed no further in this business.He hath honored me of late, and I have boughtGolden opinions from all sorts of people,Which would be worn now in their newest gloss,Not cast aside so soon.

Macbeth wants to avoid regicide, yet he caves Lady Macbeth’s bullying pressure to complete the plan. This demonstrates how easily social pressure can drive honorable, good people to commit heinous acts.
After King Duncan’s murder, Macbeth;s attitude begins to harden. He orders the murder of Banquo and tells his wife in Act 3,

"I am in blood, Stepped in so far that, should I wade no more, Returning were as tedious as go o'er."

Macbeth understands that what he is doing is wrong, but he opts to continue the bloodbath. By the end of the play, Macbeth has committed dozens of murders, lost his wife to suicide, and died in combat with MacDuff. In his play Macbeth, William Shakespeare shows how easily the best of people can descend into butchery and underscores the essential savagery of humanity.
William Golding, in his novel Lord of the Flies, takes a different approach to portraying the ugliness of human nature. By dropping presumably innocent young schoolboys on an isolated island rather than adults, Golding explores human nature free the influence of society. In a sense, this novel is a grand social experiment to ascertain the true nature of humanity! (Spoiler Alert: Humanity fails the test.) The descent of the boys into violence and savagery helps Golding make the point that the civilizing influence of society only masks the latent savage tendencies of humanity. Try as we might to erase our nature, when removed from the confines of a “civilized” environment, most people, even young innocent boys, will turn to atavism for survival.
Perhaps the most interesting way Golding highlights his pessimistic view of human nature is with the murder of Simon in the chapter entitled “A View to a Death”. Simon is the one truly “good” member of the group but is communally murdered by all the boys. It is important to note that Ralph, Piggy, and the little-uns participate in bludgeoning Simon to death during the lightning storm. Every surviving boy on the island is directly complicit in Simon's murder. The fact that Golding chooses to kill off the purest boy on the island demonstrates that he has a negative opinion of human nature.
A final example of Golding’s cynical view of human nature is in the final line of his novel. After their rescue by the British naval officer, Ralph and the rest of the boys begin to weep. Golding writes,

Ralph wept for the end of innocence, the darkness of man's heart, and the fall through the air of a true, wise friend called Piggy.

Through his experience on the island, Ralph understands the “darkness of man’s heart.” Golding believes that this darkness is an essential, ineradicable part of human nature.
I hope this helps!