Piggy, with his reading glasses and his concern about rules and regulations, represents civilization. He represents a system of law and order. When Jack gets overwhelmed with anger and punches Piggy, he is sending a punch into the heart of civilization itself. This reveals that the society on the island is disintegrating from an ordered human society into a more animalistic grouping.
Golding slowly demonstrates in this novel how man can so quickly go from reasonable to violent. There is a fine line between chaos and order. The background war of the story, materializing in the appearance of the soldier at the end of the novel, shows how thin that line is. Even civilized societies behave in uncivilized ways. The boys try to hold it together, but this punch to Piggy is just the start of the downspiral.