I've linked you back to another post that discusses this same topic.
In a nutshell, Golding is suggesting that mankind is full of basic savages. If we remove the system of reward and punishment that society has in place for us, we will resort back to the violent and primitive mannerisms of our early ancestors. Many of the characters in LOTF are cruel by nature, Jack and Roger the two most prevelent. Golding has created enough good and decent characters to give us hope, but has also made it pretty clear that those with the "right" ideas aren't heard often enough.