It is not that law covers our insticts- it simply aids to control them. We cover them with ethics and etiquette, with civility, and with our socially-learned behaviors (and our personal psychological make ups as best as we can)
The theme which drives the story is civilization vs. barbarism. The perfect scenario of good, English, privately-educated, good ol' boys facing the jungle poses the questions "how would their upbringings, their ideas of order, and their own sense of civility affect them under these circumstances?"
"'We've got to have rules and obey them. After all, we're not savages. We're English, and the English are best at everything.'" (chapter 2)
"'Maybe there is a beast....maybe it's only us.'" (chapter 5)
"The world, that understandable and lawful world, was slipping away." (ch.5)
"Which is better--to have laws and agree, or to hunt and kill?" (Ch. 11)
The answer was clear- Although they had the rules, they knew the laws, and they understood what is at stake, only Ralph and his group were able to maintain civility, though in the end you can tell it nearly breaks him.