The killing of (fall of) Piggy represents the final fall of civilization on the island. Up until that point, every bad thing that has happened came with an excuse. The birthmark face boy dying was a result of bad fire planning and excited boys; the death of Simon was a result of a wicked storm and "crazy" Simon wandering around in the middle of the night. Piggy's death, however, can have nothing but brutality associated with it. After dropping the boulder on him, there is no turning back. This is why the hunt for Ralph begins almost immediately and without any restraint.