Why does Piggy feel he has a right to be heard and what happens to him and the conch?

Piggly believed he had the right to speak because he was holding the conch. While he is speaking, or arrempting to be heard, Roger dislodges the huge rock, which crashes down on Piggy, crushing the conch shell, which he was holding, on the way. The rock pushes Piggy down a cliff, where he lands on the beach, dead.