The predominant theme for Lord of the Fllies is that without civilization humans revert to their more base, or savage natures. Author William Golding showed us through the boys who crash landed on the deserted island that people are naturally evil. Left alone without structure or rules, humans will take care of themselves first, bully others, and eventually kill.
Jack, at the beginning of the novel, couldn't even bring himself to kill a helpless pig. He had been taught by adults in his civilized society that killing was wrong. However, after the structured society the boys had built broke down, Jack (after hiding behind his painted face) not only killed a pig, but eventually killed people.
A good way to find a theme in a piece of literature is to ask yourself, "If someone asked me what life lesson I learned from reading this piece, what would it be?" Avoid summing up your answer in a predictable cliche, but use your own words to say what the work was about.
Within the theme of good vs. evil you also have the conflict of savagery vs. civility; humanity vs. barbarism, and the assasination of innocence by the basic Id, as Freud would argue, that exists within us stating that "we are all basically evil".
The main theme of The Lord of the Flies is good vs. evil. Jack represents the evil nature inherent in man, while Ralph represents the good in man. The two come to a head when Jack becomes more and more savage with the absence of rules and moral guidelines on the island. Ralph tries his hardest to maintain morals and values and to not get caught up the savage behavior, but finds that he is tempted: its just not that easy to be good! Fortunately Ralph is able to reign in his evil tendencies, and the boys are discovered before an all out war can rage.