Golding uses the boys' fear of a mythical beast to illustrate their assumption that evil arises from external forces rather than from themselves. This fearsome beast initially takes form in their imaginations as a snake-type animal that disguises itself as jungle vines; later, they consider the possibility of a creature that rises from the sea or the more nebulous entity of a ghost. When they spot the dead paratrooper who has landed on the mountain, the boys feel sure that they have proof of a beast's existence. In fact a beast does roam the island, but not in the form the boys imagine.
Golding wanted to illustrate in this novel the dark side of human nature and make the point that each member of humankind has this dark side. The boys conceptualize the source of all their worst impulses as a beast, some sort of actual animal or possibly supernatural creature inhabiting the island. Yet all along the boys take on the persona of the beast when they act on their animal impulses. There is no external beast.
Golding conveys the beast's identity through the literal actions of Jack and his tribe and through the abstract concept conveyed in Simon's vision. Simon's revelation about the beast comes upon him after he witnesses the sow's death and beheading. As an observer instead of a participant, Simon is able to comprehend the brutality of the act. The sow's head becomes covered with flies, creatures that lack the capacity to feel compassion for or empathy with the dead sow, occupied entirely by their need to eat and multiply. That compassion is one of the key dividers between humanity and animality; tellingly, Jack lacks compassion for the littluns and the vulnerable Piggy. Soon his hunters lose their compassion as well, seeking only to hunt meat and increase the numbers of their tribe or kill those who will not join.
When Simon hallucinates that the staked head is speaking to him, his perception of the other boys as the island's true threat is confirmed. The Lord of the Flies confirms that "You knew, didn't you? I'm part of you? Close, close, close! I'm the reason why it's no go? Why things are what they are?"
Note that the literal translation of the Greek word Beelzebub, a term used for the Judeo-Christian idea of Satan, is "lord of the flies," and flies feast on dead animals and excrement. When Simon asks the assembly "What's the dirtiest thing there is?" he looks for the answer "evil" but also included in that answer is decay and death. Ironically, Jack's excretory answer is partially correct.
Jack provides more insight into the beast's identity when he asserts that "The beast is a hunter," unwittingly implicating himself as part of the problem, a source of the boys' fears. His lust for power and authority causes him to commit and encourage savage acts against his own kind — an accurate measure of his depravity. Sitting in front of his tribe, "Power . . . chattered in his ear like an ape." The figurative devil on his shoulder is his own animality, looking to master other creatures.
Golding pairs the devolution of Jack's character with Simon's hallucinatory revelation to paint a complete picture of humankind's dark side — that which the boys call "the beast."
Part of Golding's intent was to demonstrate that the evil is not restricted to specific populations or situations. On the island, the beast is manifest in the deadly tribal dances, war paint, and the manhunt; in the outside world that same lust for power and control plays out as a nuclear war. Prior to the war, some of the boys, such as the perpetually victimized Piggy, experienced the brutality of others on the playground, an environment often idealized as the joyous site of a carefree childhood. Within civilized society the beast expresses itself in various ways: through acceptable venues such as the military; in unacceptable forms such as madness or criminality, which carries punitive repercussions; or concealed in the maneuvers of politics and other nonviolent power plays. In Lord of the Flies Golding illustrates that evil is present in everyone and everywhere; humankind's work lies not in the impossible mission of eradicating it but in the struggle to keep it from becoming the dominant force in our lives.