At the end of Chapter 10, Jack and his crew steal Piggy's glasses. In Chapter 11, Ralph, Piggy, and Samneric go to Castle Rock to get the glasses back. Ralph and Jack get into a fight. Jack's tribe takes Samneric and ties them up. Then Piggy tries to speak and reason with them. Holding the conch, he scolds Jack's tribe for acting like children. Roger pries a large rock loose and it falls, striking Piggy and the conch. The conch is destroyed and Piggy is sent falling forty feet onto a rock in the sea. He is killed.
Then the sea breathed again in a long, slow sigh, the water boiled white and pink over the rock; and when it went, sucking back again, the body of Piggy was gone.
In Chapter 12, Ralph has left Castle Rock and is now alone. Piggy, the most intelligent, reasonable boy on the island is dead. And this comes after the most peaceful boy on the island (Simon) has also been killed. Ralph is the only one left from his tribe. The smashed conch is symbolic of the destruction of order and civilized behavior on the island. The conch had been a tool and a symbol for order. It is destroyed with Piggy's death, this marks the completion of the devolution from civilization to savagery.