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.