At the time when William Golding wrote Lord of the Flies, the prospect of nuclear Armageddon seemed frighteningly real. Despite the remarkable advances of science and technology, the world was just a heartbeat away from total destruction, which would leave what was left of humankind in a primitive state.

It is this thin veneer between civilization and barbarism that Golding sets out to explore in the novel. The privileged young English schoolboys who crash land on a remote desert island like to think that they're the representatives of civilization, with its rules, order, and stability. Coming from such privileged social and educational backgrounds, they cannot conceive that they could be anything other than proper little English gentlemen, whatever setting they happen to find themselves in.

But as it turns out, they had barbarian tendencies lurking within their souls all along. Of all the boys, only Simon truly understands this, and he ends up as a victim of the others' wanton savagery.

Golding himself understands this point, and he endeavors to construct an entire novel out of it. In doing so, one could also say that he offers a critique of British imperialism, which was based on the same kind of complacency and sense of superiority displayed by the boys on the island.

Far from being a civilized endeavor, however, the building of the British Empire was truly barbarous, displaying far greater savagery attributed to colonized peoples. And the same could be said of the boys in Golding's story. They may like to think that they're civilized British gentlemen in miniature, but like their parents and their ancestors, they're simply deluding themselves.

In writing Lord of the Flies, William Golding wanted to show in one microcosm the evolution—or actually devolution—of society. Having served in the British Navy during World War II, Golding saw a lot of evil. Although he didn't claim a one-to-one correspondence with historical accounts (the way that Animal Farm represents certain aspects of Russian history), he did want to show several things about British and, by extension, European society.

In Ralph, the values of democracy are apparent. Ralph seeks to be a good leader who incorporates suggestions from the rank and file into his "government" and who works for the good of the society. He consistently tries to consider the good of the littluns and personally works on building the shelters. The novel also shows how lust for power can destroy a democracy—pictured by Jack's ability to lead the boys into his tribe in opposition to Ralph. Golding also wanted to demonstrate how, when forced to compete for scarce resources, people will revert to animalistic tendencies, quickly abandoning any sense of right and wrong. His insertion of Simon into the story was to show the people in society who tend toward spirituality—who have special insights that others don't have.

By creating a mini-society of boys left on their own, Golding revealed what he perceived to be the depths of human nature and sounded a warning about the need to diligently guard civilization from the brutality of which mankind is capable.

Golding once asserted in an interview that the theme of Lord of the Flies is "an attempt to trace the defects of society back to the defects of human nature. The moral is that the shape of society must depend on the ethical nature of the individual and not on any political system however apparently logical or respectable." 

If we take the above comment to be an accurate representation of Golding's intentions (and a comprehensive explanation as well), the answer to the question of why Golding wrote Lord of the Flies becomes one of philosophical commentary on human nature. He wrote the novel to prove a point (or to explore a point) regarding (1) the nature of society and (2) the flaws in human nature that contribute to the flaws in society. 

The novel might be taken to suggest that brute nature can exert itself over and against civility when a brute nature offers an equally cogent and sensible mode of interpersonal relations. Considering how well Jack does and how poorly Piggy and Ralph do, one might pause to wonder if the animalism and tribalism that Jack advocates isn't the more functional way to organize a group of boys on a wild island. 

Civilization, in a well-populated and highly regulated nation-state, is the more sensible mode of being when organization and civility provide superior degrees of safety and sustenance to people than tribalism would. On an island with no infrastructure whatsoever and no clear practical reason to be civilized, the darker side of human nature wins out--because it is as powerful as man's higher nature or because it is man's truer nature?

Regardless of our answer to the question of why man's brute nature wins out on the island, the fact remains that the ethical core for the boys is not so fully developed toward civility that it might present a valid, compelling alternative to base emotions--aggression, competition, and avarice.  

The main reason he wrote the book was probably due to his having served in the British Navy during World War II.  He saw some horrific events during his time in the Navy including the D-Day landing at Normandy and the sinking of the German battleship, the Bismarck and battles with other ships. After witnessing all that he did, he commented that "man produces evil like bees produce honey."  Since Lord of the Flies is about man's basic evil nature that is only tamed and held in check because of the rules of civilized society, it seems a logical connection to make.  Also, Golding had read the book, Coral Island, which is about three British boys shipwrecked on a deserted island having to fend for themselves and doing so peacefully in an ideal setting.  Golding, after having seen the horrors man inficts upon man, and seeing how even the "good guys" can become savage, decided to use a similar setting only showing the side of people he thought to be the true side. He wanted his boys to act as he thought real boys might act in that situation.

Why did Golding write Lord of the Flies? Why does he want to show that violence is innate in all of us?

Golding's Lord of the Flies provides an interesting companion to Orwell's 1984.  Both arise from the aftermath of the war, and both are concerned with issues of power and tyranny as well. The symbol of the Lord might find its counterpart in Big Brother, for if the latter represents an omnipotent authority that human nature fears yet is willing to accept (at times Orwell suggests even "needs"), the former represents something deep and innate also--the fear of the unknown that we convert into something "real" in order to act out or at least acknowledge the violence with in us. Both works can be understood in terms of Freud's ideas on unconscious fears and desire for and resentment of the father figure in society (and the family), and human impulse toward the "death principle."