Roger purposefully drops the rock that kills piggy. how does this detail support the theme that without anyone to enforce the rules man will break them

Since they lived on an island with no adults to tell them the right way of doing things(rules), they have been forced into the mindset of chaos, so much so that Roger killed Piggy without thinking twice. If we lived in a world without rules, we would all think that its okay to do what he did. Since we live in a world that enforces rules we cant go and do that.

CONCLUSION: We need to enforce rules so then humanity (or at least most of it ) don't break the laws and force everyone into eternal chaos. (until someone enforces rules)