What is Golding's theme in Lord of the Flies?

The predominant theme of William Golding's Lord of the Flies is that man innately operates on predatory instincts and being civilized is but a veneer.

There is a Russian proverb that states, Man is a wolf to man. Certainly, this proverb holds true in Lord of the Flies. For, throughout the narrative of Golding's work, the prevailing conflict exists between the basic instinct to be "a wolf" (the beast in man) or the conditioning...

The predominant theme of William Golding's Lord of the Flies is that man innately operates on predatory instincts and being civilized is but a veneer.

There is a Russian proverb that states, Man is a wolf to man. Certainly, this proverb holds true in Lord of the Flies. For, throughout the narrative of Golding's work, the prevailing conflict exists between the basic instinct to be "a wolf" (the beast in man) or the conditioning to be civilized.
In this narrative, well-disciplined British schoolboys are stranded on an island because the plane they have been in has been shot down by enemy fire. However, once the trappings of society begin to wear off as the boys shed their clothes, their hair grows longer, they no longer assemble and allow the one holding the conch to speak, they paint their faces, and they dance a ritualistic war dance, the "beast" (the wolf) in them emerges and gains strength in them with each savage act.

More and more, the boys regress to beastly behavior as they live a wild and barbaric life in the jungle. In fact, the savagery in them dominates to the point that the intuitive Simon, who recognizes the evil in man, is killed. Later, after Piggy is brutally murdered by the sadistic Roger, Ralph must flee for his life from Jack. Finally, Roger sharpens his spear and Jack sets fire to most of the island in his rage (Ch. 11).

When Ralph is finally rescued, he 

...wept for the end of innocence, the darkness of man's heart, and the fall through the air of the true, wise friend called Piggy. (Ch. 12)

By the end of the novel, the veneer of civilization has been completely eliminated since Jack and Roger have regressed to mere beasts who have tried to kill Ralph, once the figure of authority. Without the enforcement of civilized laws and rules, man becomes but a "wolf to man."