The death of Piggy follows a series of plot events. Ralph and Piggy decide to face Jack and force him to listen to reason. They think they will be able to talk some sense into him. Jack and the other boys still have Piggy's glasses, so they decide to confront him and get the glasses back.
Once they are at the other boys camp, Jack attacks Ralph, and they start to fight. Ralph tells Jack to give the glasses back, and they struggle to show who is in control. Piggy gives a shrill yell hoping to get the attention focused on him and off of the fighting. Roger is above on the mountain top and pushes a boulder down the side. Ralph hears it coming and is able to get out of the way. Piggy is struck by the boulder, and the conch shell he is still holding is shattered. Piggy falls off the mountain and to his death on the rocks below. Jack tries to attack Ralph, but he is able to get away.
The death of Piggy has symbolic value. It shows the death of innocence of these boys. They have given in to the wild ways of the island with no authority at all. Ralph is the only one left who has an ounce of who he was left inside of him.