One of the clearest expressions of the idea that mankind requires the strict enforcement of rules to maintain society is the quick breakdown of order on the island, even after Piggy and Ralph devise the scheme involving the meetings and the conch shell. Jack challenges Ralph, Piggy, and Simon and is successful in dividing the boys. From that point on, rules that could help preserve the boys' health and lives and improve their chances of rescue are flouted.
Disagreements erupt over when and by whom the shelters will be built, where the boys will relieve themselves, how they will get fresh water for drinking, who will maintain the signal fire, and how the boys will sustain themselves. Mostly due to Jack's unchecked egotism and influence through fearmongering, barbarity takes hold of the majority of the boys.
In chapter four, Roger gathers a handful of stones and begins to throw them near Henry, but he doesn't pelt him directly. At this relatively early point in the novel, there still exists
...a space round Henry, perhaps six yards in diameter, into which he dare not throw. Here, invisible yet strong, was the taboo of the old life. Round the squatting child was the protection of parents and school and policemen and the law.
Once it fully sinks in that there are no adults enforcing rules and demanding adherence to behavioral standards, Roger and his cohorts descend into the depravity that results in the deaths of Simon and Piggy. It takes the appearance of the naval officer, an embodiment of law, rule, and discipline, to prevent Ralph's death.