For me, the central most important passage of the novel is at the very end. Firstly, there is the breathtaking moment which really cements the theme of the whole novel: a moment at which the characters we have forgotten the age of once again become little, little boys. Ralph, Jack - sharpened sticks - these boys are 12 and younger:
He staggered to his feet, tensed for more terrors, and looked up at a huge peaked cap. It was a white-topped cap, and above the green shade of the peak was a crown, an anchor, gold foliage. He saw white drill, epaulettes, a revolver, a row of gilt buttons down the front of a uniform. A naval officer stood on the sand, looking down at Ralph in wary astonishment.
For the first time in the novel, Golding's narration twists perspective - where before everything tends to be described for a little boys' eyes, seen as tall, large, and scary - we suddenly see things from the naval officer's eyes. And the novel's logic really does cohere - these horrors have been brought about by children:
A semicircle of little boys, their bodies streaked with colored clay, sharp sticks in their hands, were standing on the beach making no noise at all.
“Fun and games,” said the officer.
If only he knew. And then the boys all begin to wail, just like little boys might. And in the middle of it all, Ralph's thoughts suddenly become hugely clear: he realises the "darkness of man's heart", and, heartbreakingly, the true importance and friendship of Piggy:
And in the middle of them, with filthy body, matted hair, and unwiped nose, Ralph 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.
While the denouement of the novel is very telling, perhaps the most significant passage in "Lord of the Flies" is in Chapter 8 where Simon encounters the pig's head and utters, "I know that." It is at this moment of transcendence that Simon understands "the darkness of man's heart." This passage then, is pivotal to the plot as well as to the development of such themes as Good and Evil and Reason and Emotion. For, while Piggy who represents reason often has moments of ineffectiveness and Ralph who recognizes the power of thinking succumbs to some of excitement in the rituals of the hunters, only Simon, who is intuitive, reaches the spiritual knowledge necessary to man's survival. He alone understands mankind's essential illness.
The great achievements of science (represented by Piggy) and the irrational emotions of warring powers (Jack, et.al.), Golding felt, spell doom for human nature. Thus, Piggy, the rational animal, has the same nomenclature as the hunted animals, the pig, as well as "the beast": the pig's head.
Only Simon who comes down from the mountain to give the boys this message of "the evil men do" understands, for he is the only spiritual character in the novel. He is the Emersonian hero, who is self-reliant and a loner, who communicates with nature, and who observes and understands. Yet, he becomes the sacrificial lamb at the hands of the brutish boys from whom reason has fled. Piggy's glasses have been broken and soon he will die. Ralph, too, is hunted and only saved by the intervention of fate in the form of a naval officer. But, the officer glances at the warship to remind the reader that without the Simons of the world, judgment will often have "fled to brutish beast/and men [lose]their reason" as Marc Antony said in Julius Caesar.