When thinking about the overall significance of this amazing story you will want to think of how quotes reveal what Golding is trying to say about humanity. Of course Golding is writing about opposition. The central opposition in the novel is that between the forces of civilisation and savagery, or order and chaos. Golding explores the competing instincts that dwell within us all: to live by rules, obey morals and act for the greater good of society, and then the opposite side, which is the desire to dominate, enforce one's will and act immediately to gratify desires.
This conflict is explored throughout the novel through examining the boys' gradual slide into lawlessness as they adapt to life in a barbaric jungle away from the normal controls on their behaviour (law, parents, school etc). These two forces are represented by the two characters Ralph (civilisation) and Jack (savagery).
Golding's conclusion is that the instinct for savagery wins out in the end. It is far more primal and fundamental to us than the instinct of civilisation, which he sees as a result of social conditioning rather than any moral goodness within humanity. We can see this through the example of the boys: when left to their own devices without any external forces of control, the instincts for savagery win out, even in the defender of civilisation, Ralph. The concept of the innate evil within all of us is central to understanding this novel, and is symbolised by the beast and the sow's head on the stake. This is the realisation that causes Ralph and the other boys to weep at the end of the novel, ironically when they should have been most happy: "Ralph wept for the end of innocence, the darkness of man's heart, and the fall through the air of teh true, wise friend called Piggy." So the "darkness of man's heart" is therefore the central moral of this story.
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.
So, there you have it - according to Golding, we are savage, evil creatures, only barely kept in check by civilisation. When those restraining influences are removed - well, you have read the book, so you know what can happen!