What is the message ? And how have the setting help covering this

The central concern of Lord of the Flies is the conflict between two competing impulses that exist within all human beings: the instinct to live by rules, act peacefully, follow moral commands, and value the good of the group against the instinct to gratify one’s immediate desires, act violently to obtain supremacy over others, and enforce one’s will.

The setting makes this message possible by leaving the children accountable only to themselves--which reveals the struggle that is so repressed in a more "civilized society."