Firstly, let's talk about the symbols in this chapter.
Piggy is the symbol of civilised life and actions. He is overweight, ill with asthma (ass-mar as the boys call it) and has poor eyesight. He been able to overcome these shortcomings somewhat through the inventions of the modern world (symbolised through his glasses). Throughout the novel, it is Piggy who reminds the boys to remember their civilised, human selves, by insisting that they treat each other with respect by calling everyone by their name and by creating a structured and respectful way to discuss and solve problems through the use of the conch. Ironically, Piggy is one of the only boys who does not receive the same treatment he has demanded for the others. He remains "Piggy".
Piggy's glasses are a symbol of the innovation of the modern world - where problems can be overcome with intelligence and creativity. In this world, all people can be equal and the weak are protected. This is in stark contrast to the savagery of more primitive life - where violence and strength are used to overcome adversity and where weakness and soft-heartedness are scorned and can lead to death. Ralph's "camp" is robbed of the glasses by Jack, Roger and the others, so that they will be successful with fire in their new camp with the Lord of the Flies. Without the glasses, Ralph, Piggy and the boys are unable to maintain their signal fire, which is their hope of being rescued by adults.
The Conch is the symbol of a life of order and respect. It is used to gain the attention of the group, and as a turn-taking tool for group discussions. Both of these actions are part of an ordered, safe society. It is smashed by the boy least able to understand civilised ways - namely, Roger, who takes pleasure in hurting and destroying.
The Lord of the Flies (the pig's head) is a symbol of the savagery that the boys have found within themselves and created around them. It was discovered by Jack after the boys had slaughtered a sow suckling her young. The Lord of the Flies allows Jack to become a religious leader for the boys, in which role he makes them follow quasi religious rituals in honour of the Lord of the Flies. The Lord of the Flies is destroyed by Ralph, who is arguably the least savage amongst the leaders.
Piggy is killed by the boulder that Roger rolls down the side of the hill. In line with his non-athletic nature, Piggy is unable to escape the rock and is struck down, and rolls off the side of the mountain.
His body floats away from the savagery of the island to the peace of the sea.
Simon, who is the one character who has discovered that the "Beast" is not a real being, but rather the evil that lurks within all of them. Simon is murdered by the boys who mistake him for the beast. His body is washed into the sea in a violent storm, symbolises an act of cleansing away the the goodness in Simon from the evils that have grown on the island.