In the novel 'Lord of the Flies' by William Golding, by the time the readers get to the part where Piggy is hit by a rock and falls to his death, the author knows they will have identified with him a little so their feelings will be heightened by his sad death at such a young age. We, as readers, have seen poor Piggy's fumbling and tentative attempts to be Ralph's friend, his sensible comments and his developing wisdom and good sense. Each one of us has some small thing we are conscious about with our own selves and so we feel for Piggy with his sweaty podginess or short-sighted blinky eyes. William Golding emphasizes and lingers over Piggy's death because he knows it will affect us very deeply and that is what he wants. It's a shame the boys have,conversely, become desensitized to the suffering of others by now.
In my opinion, Piggy's fall (this is in Chapter 11) after he has been hit by the rock is emphasized because it symbolizes what has happened to the boys as a group.
We talk about people "falling" when they go from good to bad -- thus, we talk of the "fall" of Adam and Eve. I think that what has happened to the boys during their time on the island is that they have fallen. They started out relatively civilized but have fallen to the point where they exult in killing.
So I think Goldman has Piggy die in this way, and lingers over his death a bit, to emphasize what has happened to the boys.
Piggy represented goodness and civilization. Roger and Jack represented the opposite. When Rodger hits Piggy with the rock and causes him to die the boys do not even react to the murder. When Simon had died the boys had accidentally killed him in frenzy and with fear. They had continued to attack him, but the next day they could not even look a each other because of shame.
By the time Piggy is killed the scene lingers to show that the boys are almost numb to death. They have lost all degree of the doctrines of civilization. I believe that the scene lingers to show how little emotion the boys have left. They no longer even value human life. It also gives the reader the chance to realize that if they could kill Piggy, they will probably kill Ralph.