Piggy is not killed for a reason while the wild pigs are killed for sustenance. Even if the boys did needto hunt in order to survive, killing the pigs was at least nearly practical. The murder of Piggy was not practical at all, merely vengeful.
Connecting Piggy to the pigs is a stretch for me. Perhaps the strongest connection for me is the image of the boards head on a stake - the Lord of the Flies. This image is a demonstration of the violence which the boys are capable of, portending the ultimate violence done against Piggy.
Agreed. I always wonder if things might have been just a little different if Piggy had been called something else. He was the one who wanted to maintain order and discipline and rules and civility--all things in which the boys were completely disinterested. He was physically unappealing--his size, his asthma, his glasses, his tuft-like hair, his untanned skin--but it was his ideas which got the most negative reaction. The irony of the pig hunts is not lost on Piggy, who understands quite well his position as a target for the "savages." In the end, they do their worst and any vestige of civilization is lost with him.
When Piggy and Ralph first meet, Piggy requests that he not be called what he was in school, but Ralph immediately does just that, and even introduces him as Piggy, so the boy is again stuck with the name. This act signifies that Piggy will never be taken too seriously. After all, his shape and condition are things often ridiculed by other boys.
Of course, the boys make the tragic error of not listening to the rational boy who looks more like an adult. If any character represents the civilized world that they have left, Piggy is the one. Ironically, then, his name is that of the feral beasts that the boys hunt and eventually slay. Like the feral pigs, Piggy is reduced to little more than a hunted creature, who is brutally murdered as Roger hurls the pink granite rock upon his head, sending him dashing against many rocks down into the sea that washes him away. Piggy is made the sacrificial animal that satisfies the evil and sadistic nature of the boys, especially Roger.
Despite his intelligence andimportant fire-starting glasses, Piggy is not taken seriously by most of the boys. They cannot get past his weight and wheezing. Like the pigs that the boys kill and eat, Piggy is a life-sustaining force (ideas and glasses); also like the pigs, he is fat and ultimately expendable. The boys eventually pay no attention to any of his ideas and just keep his glasses.