This is a big question about a big novel with some massive issues! However, to respond, one of the key messages that this book gives is about the evil that is within humanity and how it is expressed. What I will do is talk about three important parts of the novel where this is developed. Hopefully this will give you a basis for understanding the rest of the novel and how it reflects this theme.
One of the key quotes you will definitely want to look at 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.
I hope this helps. I have added links to the summary of the novel in enotes and other helpful links that should help. Good luck and I hope you enjoy your study of this excellent story!