The conch shell is used by Ralph to call everyone to order when the boys have their meetings. He understands very well the necessity of establishing a system of rules on the island if the boys are not to fall into bloodshed and anarchy. In due course, the conch comes to symbolize the civilized order, with the promotion of justice it entails. The conch confers authority upon whomever holds it, and at first, that authority is respected. But Jack hates and resents Ralph and wants nothing to do with any kind of civilized order. He wants to establish a dictatorship on the island with himself as dictator. His power will be based purely on brute force; he doesn't need the conch and all that it represents. In Jack's world, justice is just so much can't; only might is right.
So long as the conch exists, it holds out the prospect, however remote, that some measure of justice can be secured, irrespective of the myriad conflicts that routinely break out among the boys. But when the conch shell is smashed—with Piggy killed at the same time—all hope of achieving justice is shattered with it.