Thoughts on Lord of the Flies

I finished Lord of the Flies by William Golding today. It's amazing. One of the best books I've ever read. If not the best book I've ever read. Easy to read, yet quite deep and very true. It portrayes the darker side of human nature in a very fitting way, I think. Also very nice and not too overdone use of metaphors.
For those of you who haven't read the book and are too lazy to read the wikipedia article: It's essentially about a group of boys who are by themselves on a deserted island. While they act quite civilized in the beginning, things turn bad and some of the boys are harmed. I won't tell you more in case you want to read the book and don't want to be spoiled. Anyway, I could go on about this book forever, but the book is only the starting point of my thoughts.
As I said, the book is very true. We can look at psychological experiments (e.g. the stanford prison experiment ) or things that happen in real life (e.g. raiding, torturing and raping in times of war) to see that the book is not a far-fetched story that would never happen in real life. It seems that there is a part in (almost?) every human being that wants to seize and exercise power, that wants to hurt and kill and humiliate. Usually this beast inside each of us is kept in check by society, by culture, by civilization or by law. But if certain circumstances arise - either because nobody else is around or because it's a situation to which the normal rules don't seem to apply - this beast suddenly becomes freed and takes control. This is scary, but not a very new thought to me. However, what I'm wondering is this: If we really are like that, then why don't we all live like that? Why did societies evolve almost everywhere, where there are rules that tell us not to kill etc. As far as I know all of the major religious groups have rules like that and even though death penalty is still legal in some parts of the world, just killing someone because he/she pisses you off is not. Why?
A couple of answers come to my mind (that most people are better off in a society that provides safety for (almost) everyone etc.) but no matter which answer, the next question arises: If people really believed that a system like that is better for everyone or whatever the reason might be - why do they get rid of all these rules so willingly, so quickly, if they get the opportunity?