It is fitting that the conch is destroyed at the same time as Piggy's death. The conch is the object that established some kind of order on the island. It is the tool the boys used to gather together and discuss strategies for survival. Therefore, it also is the tool used to establish unity. The conch also provided opportunity for anyone to speak. Jack came to despise the conch because he thought only certain people should speak. Jack is the main reason for the disruption of that order and that unity. When the boys split into two tribes, the unity and order is broken. It is fitting that Piggy, who had been a voice of intelligence and reason, still tries to use the conch to reestablish order in Chapter 11.
Ralph tells Piggy that Jack will hurt him if he tries to use the conch to make his point and get his glasses back. Piggy responds by saying that Jack can't hurt him any more than he already has:
"I'm going to him with this conch in my hands. I'm going to hold it out. Look, I'm going to say, you're stronger than I am and you haven't got asthma. You can see, I'm goin' to say, and with both eyes. But I don't ask for my glasses back, not as a favor. I don't ask you to be a sport, I'll say, not because you're strong, but because what's right's right.
Piggy still believed, or at least hoped, that the conch could still be used to establish order, that Jack and his followers were still capable of doing the right, civilized thing. Piggy continues to seek justice, to do the right thing. The conch symbolized order and fairness. That's why it is fitting that they are both destroyed together. Piggy pleads with Jack and his tribe but they are too far gone into savagery. Piggy is killed and Jack responds, not by acknowledging Piggy's death but by acknowledging the destruction of the conch. With Piggy and the conch destroyed, a of voice of reason and the symbol of order are both destroyed.