While the boys' initial perception of the island is that of a type of Garden of Eden, the flaw in this idyllic island is the preexisting condition of evil inherent in the boys. Golding hints at this innate evil in the passage of Chapter 4 in which Henry sits on the shore, delighting in his power to control the
transparencies that came questing in with the water over the hot, dry sand...Like a myriad of tiny teeth in a saw, the transparencies came scavenging over the beach....He poked abut with a bit of stick...and tried to control the motions of the scavengers....He became absorbed beyond mere happiness as he felt himself exercising control over living things... [which] him the illusion of mastery. He squatted on his hams at the water's edge, bowed, with a shock of hair falling over his forehead and past his eyes, and the afternoon sun emptied down invisible arrows.
With this foreshadowing of conflict and killing, Roger, who keeps "to himself with an inner intensity of avoidance and secrecy" waits and watches furtively behind a great palm. When a wind knocks nuts from the palm onto the ground, Roger picks up a stone--"that token of preposterous time"--and bounces it a few yards to the right of Henry. Roger gathers more and throws them near to Henry:
Here, invisible, yet strong was the taboo of the old life. Round the squatting child was the protection of parents and school and policemen and the law. Roger's arm was donditioned by a civilization that knew nothing of him and was in ruins.
When Jack calls his name, Roger opens his eyes and sees him: "a darker shadow crept beneath the swarthiness of his skin; but Jack noticed nothing." This "darker shadow" is the innate evil of Roger which is already on the Garden of Eden island.
With the restrains of civilization gone, this evil then emerges. The symbol of the rock carries this motif through Chapter 6 when Jack speaks of a large rock making a bridge across the island into the jungle. As they cross, the intuitive Simon feels "a flicker of incredulity...[he] thought of the beast...before his inward sight the picture of a human at once heroic and sick." Later, Golding describes the water sinking among the rocks:
There was one flat rock there,...and the waters sucking down on the four weedy sides made them seem like cliffs. Then the sleeping leviathan breathed out, the waters rose, the weed streamed, and water boiled over the table rock with a roar. There was no sense of the passage of waves; only this minute-long fall and rise and fall.
A profoundly symbolic passage, this suggests the rising of the stone-age primitive, ("leviathan"-whale-suggesting "huge and "powerful" with Biblical connotations) evil behavior of the hunters which will end in death as foreshadowed in the earlier passage about Henry and Roger. Carrying the symbolism of the rock through the novel, the final chapter (12) has many references to this symbol. One significant example is "death rock flowered again." And, of course, Piggy is brutally murdered in Chapter 12 as his head crashes against rock. Even before his death, Piggy hears on top of Castle Rock voices raised where "the horrors of the supernatural emerged." The inherent evil in the boys which has initially invaded their idyllic island emerges to wreak destruction.
One of the key quotes you will definitely want to talk about is: "Ralph wept for the end of innocence, the darkness of man's heart, and the fall through the air of a true, wise friend called Piggy", which comes towards the end of the novel (Chapter 12) and comes after the appearance of the navel officer that is going to rescue the boys. This is a key quote, because although Ralph should be happy because he is going to be rescued and returned to civilisation, what has happened to him and the boys on the island has made him aware of the evil within humanity and the savage instincts that are latent within all of us. He has lost his innocence, and will never be the same. This is the reason for his grief and tears.
Another key quote comes from Simon when the boys are discussing The Beast and if it is real or not: "There isn't anyone to help you. Only me. And I'm the Beast . . . Fancy thinking the Beast was something you could hunt and kill! . . . 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 the way they are?” Simon is the first boy to realise that the evil summed up in the beast isn't actually external - but internal, based in themselves. This is something that he further realises when he confronts the Lord of the Flies later on in the novel.
Lastly, when Jack has killed his first pig, Golding writes: "His mind was crowded with memories; memories of the knowledge that had come to them when they closed in on the struggling pig, knowledge that they had outwitted a living thing, imposed their will upon it, taken away its life like a long satisfying drink." This quote clearly establishes that Jack is attracted to the killing of pigs not because of the need to feed the boys but because of the joy of letting his primal instincts loose and the desire to impose his will and strength upon another creature.