The destruction of the conch, and the death of Piggy symbolize the destruction of what shred of civilization was left on the island. Piggy's commitment to rules and law and order has resulted in his death, and Ralph has proven to be powerless before Jack, who, it is clear, will rule by force. Piggy's speech just before Roger strikes him with the rock makes the point clear:
Which is better—to be a pack of painted Indians like you are, or to be sensible like Ralph is? . . . Which is better—to have rules and agree, or to hunt and kill? . . . Which is better, law and rescue, or hunting and breaking things up?
Roger's rock can be understood as an answer to this question, and it is clear that the twisted vision offered by Jack and Roger has supplanted reason and order for good.
As civilization on the island erodes and the boys descend into savagery, the conch shell loses its power and influence among them. Once a symbol of order, the conch no longer exists, nor does the order it establishes on the island. It was all consumed by the savagery of mankind. At first it was a discovery that seemed like a gift from heaven. Something that allowed the boys to remain children at heart. However, after time the shell’s importance began to slowly collapse until became nothing but a shell. And when it is destroyed, along with it goes the civilization the boys once lived with.