What are the similarities and differences between the death of the pigs and the death of Piggy?
On one level, there is obviously a parallel to be drawn. Golding, by virtue of his name, compares Piggy (who likes eating) to pig (which is eaten), and there's a couple of moments in the novel where that comparison is made explicit:
“I don’t agree with all Jack said, but with some. ’Course there isn’t a beast in the forest. How could there be? What would a beast eat?”
“We eat pig.”
Moreover, when Piggy dies, Golding's narrative seems to be seen from one of the boys' perspectives, likely Ralph's. There is something very childish, sickeningly unknowing about "stuff", and "like a pig's": it is a description which does not take in the full import of the event:
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.
Piggy is of course compared to a "pig", and not even given time to "grunt". But, beyond that, I think his death is quite different from a pig slaughter. Roger kills Piggy alone, not in a group: it is a deliberate act of sadism. It is quick, silent, and immediately deadly. The pig killings are done in a group, they are noisy, and they take some time:
“Make a ring!”
The circle moved in and round. Robert squealed in mock terror, then in real pain.
“Ow! Stop it! You’re hurting!”
The butt end of a spear fell on his back as he blundered among them.
They got his arms and legs. Ralph, carried away by a sudden thick excitement, grabbed Eric’s spear and jabbed at Robert with it.
“Kill him! Kill him!”
All at once, Robert was screaming and struggling with the strength of frenzy. Jack had him by the hair and was brandishing his knife. Behind him was Roger, fighting to get close. The chant rose ritually, as at the last moment of a dance or a hunt.
“Kill the pig! Cut his throat! Kill the pig! Bash him in!”
Ralph too was fighting to get near, to get a handful of that brown, vulnerable flesh.
The two events are entirely different in the way they are described and the mood presented. Yet the key point, I think, is that Piggy is seen through such an unforgiving, bestial description: "like a pig's". This sensitive, wise boy is killed as a dumb, fat animal.