Why is Piggy’s death equated with the slaughter of the pigs?

I'm sorry, I must be missing something in the text. I do not see the section of the text that equates Piggy's death with the slaughter of pigs. Do you have a quote available?

"Behind them on the grass the headless and paunched body of a sow lay where they had dropped it." page 176

"Piggy's arms and legs twitched a bit, like a pig's after it has been killed." Page 181.