Golding once asserted in an interview that the theme of Lord of the Flies is "an attempt to trace the defects of society back to the defects of human nature. The moral is that the shape of society must depend on the ethical nature of the individual and not on any political system however apparently logical or respectable."

If we take the above comment to be an accurate representation of Golding's intentions (and a comprehensive explanation as well), the answer to the question of why Golding wrote Lord of the Flies becomes one of philosophical commentary on human nature. He wrote the novel to prove a point (or to explore a point) regarding (1) the nature of society and (2) the flaws in human nature that contribute to the flaws in society.

The novel might be taken to suggest that brute nature can exert itself over and against civility when a brute nature offers an equally cogent and sensible mode of interpersonal relations. Considering how well Jack does and how poorly Piggy and Ralph do, one might pause to wonder if the animalism and tribalism that Jack advocates isn't the more functional way to organize a group of boys on a wild island.

Civilization, in a well-populated and highly regulated nation-state, is the more sensible mode of being when organization and civility provide superior degrees of safety and sustenance to people than tribalism would. On an island with no infrastructure whatsoever and no clear practical reason to be civilized, the darker side of human nature wins out--because it is as powerful as man's higher nature or because it is man's truer nature?

Regardless of our answer to the question of why man's brute nature wins out on the island, the fact remains that the ethical core for the boys is not so fully developed toward civility that it might present a valid, compelling alternative to base emotions--aggression, competition, and avarice.