Man, thinking about that scene still gets me tense. Piggy's death happens in Chapter 11, 'Castle Rock,' right after Jack's tribe steals his glasses. He goes with Ralph to confront them, holding the conch, trying to make them see reason. Roger, that absolute monster, leans on a lever and dislodges a huge rock. It strikes Piggy, the conch shatters with him, and he's thrown off the cliff into the sea.
It's the most violent, definitive end to any idea of order on the island. The conch breaking is the final death of democracy and civilized debate. Without it, Ralph has no symbolic power left. Piggy was the voice of logic and intellect, however annoying the others found him. His murder isn't an accident of hunting like Simon's; it's a deliberate, cold act by Roger, showing the tribe has fully embraced brute force over any form of discussion. The last vestige of the adult world they came from just gets wiped off the rock.