Piggy represents the intelligent, civilized mind. When he is killed, he is trying to intellectually reason with the boys. He has to be killed, because he's making too much sense. Piggy is the one with the special knowledge, for instance how to blow the conch and summon the other boys. Once Piggy dies, all reason dies with him. His death symbolizes the end of reason. When people function without reason, they move to the level of id and destroy one another. In a symbolic way, Golding is saying that when the people of a society stop thinking and just react, the end of that society is at hand.