One of the strongest themes in this novel concerns the savagery that lies dormant in all of us. Even innocent boys can fall into brutal and violent modes of behavior given the proper circumstances.
This hidden tendency exists in the same people who, given better circumstances, are content to follow the rules, to treat others respectfully and safely, and to be entirely civilized. People are capable of being governed by both emotion (savagery) and reason (civility).
So, to articulate this theme as a moral lesson, we can say that Golding is expressing a warning. Human nature is not innocent, even in children. Children, as all humans, possess qualities of brutality and the moral life is not the only "natural one" for people to live.
Violence is just as natural and just as human as any other mode of living. The moral, again, is a suggestion that humans have two sides to them, as exemplified by the characters in the novel.
Most characters in the story show elements of both reason and emotion.
Back at home, the boys followed the rules and acted civilly. On the island, the learned modes of behavior (civility) gave way to the other, darker side of human nature.
As the author once commented, "the moral is that the shape of a society must depend on the ethical nature of the individual and not on any political system."
People are not all bad and they are not all good. People are both good and bad, capable of acting upon either side of their nature, capable of being ruled by either side, depending on the circumstances.