Who killed Piggy, and how did Piggy die?

Roger killed Piggy when he dislodged a giant rock.

High overhead, Roger, with a sense of delirious abandonment, leaned all his weight on the lever.

The rock struck Piggy a glancing blow from chin to knee; the conch exploded into a thousand white fragments and ceased to exist. Piggy, saying nothing, with no time for even a grunt, traveled through the air sideways from the rock, turning over as he went. The rock bounded twice and was lost in the forest. Piggy fell forty feet and landed on his back across the square red rock in the sea. His head opened and stuff came out and turned red. Piggy's arms and legs twitched a bit, like a pig's after it has been 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.