Throughout the book, Piggy represents not just civilization and rational thought but also loyalty. He has worked throughout the story to support Ralph as Ralph also worked to carry out some of Piggy's ideas and valued his input in a way no one else did because of Piggy's physique and inability to stand up for himself.
When the boulder smashes him and his glasses, it symbolizes the end of any possibility of reason or rationality triumphing over chaos and fear. The boys have completely given themselves to the beast and soon Jack is leading the boys on a hunt to kill Ralph, setting fire to much of the island and devolving completely into depravity and madness.
Jack's desire to rule over a group of "hunters" that does nothing but hunt to satisfy their desire is finally completely realized and he has complete power over the boys on the island.