How is evil exemplified in Lord of the Flies and Macbeth? What are its sources, and how is it manifested in both?

This particular writing prompt correctly indicates that both Lord of the Flies and Macbeth deal thematically with evil. You could probably go multiple directions with this answer. One direction would be to explore how the characters demonstrate their propensity to do something that most readers would consider is evil. I think it is safe to assume that intentionally killing a person is evil. What is great about both stories is that the main characters aren't rushing into the evil that is murder. Macbeth actually talks himself out of it until Lady Macbeth browbeats him into changing his mind and going through with it. Once Macbeth initiates himself into his new murderous life, the subsequent killings come much easier for him. The boys on the island are in a similar situation. Readers will actually see Jack fail to kill the pig the first time; however, by chapter four, he has worked up the courage to go through with the kill. He quickly descends into becoming a complete savage with no qualms about trying to kill Ralph. Interestingly, it isn't Jack that actually succeeds in killing a human. Roger kills Piggy by rolling a boulder onto him.
Another comparison that could be made between the two works is how the evil grows as the story continues, and characters get more and more familiar with embracing the evil. The descent into evil savagery is often a topic of discussion with Lord of the Flies, and Macbeth spirals deeper and deeper into a murderous web in order to keep himself on the throne. Both stories show how evil so completely blinds a person to the point where that person is willing to hurt or even kill former friends. This is how and why Simon is tragically killed. The boys are so focused on a possible beast, that they don't recognize a friend. Finally, I think both stories accurately show how evil is a force within every person. This is a key detail that Simon learns about who and what the beast actually is. He is the beast as much as Jack and Ralph are. All of the boys have the potential to do great evil. Macbeth is the same. His unrestrained ambition overpowers his morality, and that shows readers that the evil was always present.