The novel focuses on an internal human battle between the need for civilization and the countervailing drive toward savagery. The side that leans toward civilization feels the need or desire to live an ordered life guided by rules and a basic respect for others. The side that leans toward savagery is only interested in self-fulfillment of basic needs. This side is willing to use any means, including violence, to reach a desired goal.
The boys land on the island as civilized British school boys. They are accustomed to school rules, uniforms, and respect for authority. However, they live in a seemingly civilized world that is in the midst of a brutal, savage war. They have left England to escape war and crash-land on the island because their plane was shot down. When some boys revert to savagery, they can be seen as replicating what they have witnessed in the adult world.
Ralph and his crew represent civilization. They create rules, live by them, and insist others do the same. Their only goal is to get off the island and back to the adult world. Jack and the hunters revert to primal savagery. In part, this descent is a degeneration of the values of an English boys' public school. In part, too, the fall to savagery reflects the boys' search for a more satisfying order than Ralph offers in tribal belongingness, the appeal of rituals such as the dance and the killing chant, and the promise of protection from nameless fear through apparent displays of strength. At the same time, they develop bloodlust they first take out on a pig and very quickly turn on other boys. Jack and the hunters focus on satisfying their basest needs.
Neither side can understand or appreciate what those on the other are feeling, and acts accordingly. Just before Piggy is murdered he tries to explain that Jack and his crew need to live by rules. Meanwhile, Jack sets the island on fire to destroy Ralph, not considering that this will also destroy the fruit trees, the boys' main source of food.
Lord of the Flies is a coming-of-age story, or bildungsroman. When the boys arrive on the island, they are innocent school boys. That innocence is illustrated by their reactions to the lack of adult supervision as well as on the island exploration walk that Ralph, Simon, and Jack take. At first, the boys look forward to having fun on the island until the adults come get them.
As their time on the island grows longer, the boys change. With their long hair, unkempt bodies, and painted faces, the boys discover and act out different sides of themselves. Many of the boys develop bloodlust and ultimately commit terrible acts, including murdering Simon.
When the naval officer arrives on the beach in the last chapter, Ralph cries and the others join him in despair. They recognize the evil within themselves and humanity at large. They are no longer innocent school boys who simply want to have fun.
As the text demonstrates, Golding believed everyone has the capacity for inhumanity and evil. As the novel proceeds, it is easy to forget that the oldest of the protagonists is only 12 years old. They are mere boys. Jack, who comes to epitomize savagery, is seen as "a little boy" by the naval officer who finds them. The boys' youth gets lost in their horrific actions. One might think a child would never do such awful things. However, everyone, children included, has evil within. Even Ralph and Piggy, who act reasonably and yearn for civilization, show their capacity for evil as they participate in the murder of Simon. This act is just one of a few times that Ralph shows this capacity. Even the littluns, who are as young as six, show this propensity toward evil. When they gather among themselves, they show the capacity to act cruelly to each other and the land around them. Indeed, that the characters are boys underscores Golding's attempt to portray their decline into savagery as fundamentally representative of human nature. They are not adults who have been fully socialized and can blame their evil on that socialization. Instead, they act on natural human impulses.
As the Lord of the Flies, which represents this evil, says to Simon, you can't kill the evil within because it's part of you.