William Golding's Lord of the Flies is a symbolic novel. It is set on an uninhabited tropical island, so immediately Golding has made the world of this novel a microcosm (small world) for something bigger. In other words, what happens on this small island is a picture of what is happening in the rest of the world on a larger scale.
The characters of this novel are English schoolboys who are the sole survivors of a plane crash. These characters are all young (and therefore closer to innocence) and presumably well disciplined (as proven by Jack and his choir boys); they are not hooligans who are accustomed to causing trouble. These boys know how to follow rules and obey authority, qualities which should keep them "civilized" longer than most.
What happens throughout the novel, though, is that these civilized boys quickly divest themselves of their uniforms and their respect for the authority they elected (Ralph). Soon--much, much too soon for a group of disciplined children--things on the island turn into a fight for survival. Golding refers to the boys as savages, and their behavior fits his description.
Simon and Piggy are murdered, and Ralph is the next to be targeted. If help, in the form of a naval commander, does not arrive, Ralph would not survive. In the final chapters, it is clear that there is someone even more inclined to savagery than Jack; if the boys are not rescued, Roger would probably have established a horrific reign of cruelty and death.
While there are many potential sub-themes to this novel, Golding is clearly making the statement that human nature, without the restraints of civilization or authority, will devolve into savagery. Rather than being elevated by freedom, human nature becomes more base and evil. Every character and every movement of the plot serve to advance this theme, and the fact that is a microcosm suggests Golding's view that this is a universal truth.