This ambiguity is part of the story. The idea that children will inevitable behave like adults (even killing each other) is an indication that humans are inevitably bad or children learn behaviors like this from adults. There is a type of madness in the world...where does it come from? We all have to come to grips with this at some point...whether as adult or children. The killing of Piggy is an action that is distilled in the story. When we look at killing in context of the whole world, we sometimes dismiss it as a "necessary evil"....like in the case of war. But, here on an isolated island, it is laid bare and we must grapple with it.
