In his novel Lord of the Flies, William Golding places a group of boys on a deserted island without any authority figures or laws. Even more, these are English schoolboys who have, presumably, been well trained in following rules. What happens during their time on the island suggests several rather obvious themes Golding must have intended when he wrote.
First, as the eNotes site below states, is the battle between good and evil. These boys are good and obedient from the beginning of the novel (perhaps with the exception of Jack). Many of them try to be helpful and are concerned about the youngest boys among them (the littluns). They try to work together, though they are often distracted, just as most children would be. Roger, who becomes one of the cruelest boys on the island, still feels a sense of restraint in chapter four when he cannot quite bring himself to throw rocks directly at one of the littler boys; "invisible yet strong, was the taboo of the old life." What evolves, though, is a pack of hunters (led by Jack) who eventually kill Simon and Piggy and nearly destroy the entire island (with a fire that ironically saves them). Without the restraints of authority and law, evil wins.
Second is the theme which pits mind, body, human nature, and spirit/soul against one another. Each of the four main characters embodies one of these characteristics: Piggy is the thinker (mind). Ralph is the physical one (body), Jack is the reflection of man's tendency to do evil (human nature), and Simon is the sensitive seer (spirit/soul). One by one, these four begin to fall. Simon, representing the spirit man, falls first; he is followed by the intellect, represented by Piggy. The final showdown is between Ralph and Jack, the physical man against the depraved nature of man. If they had not been rescued, Ralph would undoubtedly have been the next to die. This is a depiction of Golding's overarching theme that when men (boys, all of us) do not have the rule of law to restrain them, evil wins.