Themes

Lord Of The Flies, because it is an allegorical novel, has many themes. Themes are recurring topics and subtexts in a story that often are used to emphasize a message the author was attempting to communicate with the reader.

Civilization vs. Savagery
At the beginning of the novel, the boys crash-land on a deserted island, conditioned to laws and morals and manners. As the novel goes on, the boys slowly realize that there are  no rules because there's not a single authority figure on the island apart from themselves. Eventually, the boys devolve into little more than animals, killing each other left and right and forming tribes.

Fear
Most of the conflict in the novel is fear-driven. When the Beast is mentioned and the boys go hunting for it, they do so out of fear. Obviously, the only thing they should have been worried about and fearing was themselves, but they seem to have misplaced their feelings.

Religion
To start, Lord Of The Flies is a literal translation of the Hebrew word Beelzebub, which is the name of a demon in the Bible. Simon is thought by many to represent Jesus Christ, since he was killed for no reason and was the only truly innocent one on the island. The scene in which Simon speaks with the sow's head (The Lord Of The Flies) is supposedd to represent the Tempation of Christ. Some might say that the island represents the Garden of Eden, pure and untainted before humanity arrived and fouled it with evil and selfishness.