How does the slaughtering of the mother pig foreshadow what is to come in Lord of the Flies?
You could look at this from a few different perspectives.
First, Piggy is eventually slaughtered by the Jack's crew. It seems to happen accidentally, but it certainly happens without remorse. Piggy's name probably contributes to the eventual slaughter of himself since the quest of Jack's boys throughout the story is to hunt and kill pigs. Having watched Jack's treatment of Piggy the entire time, he has certainly been in the process of at least mentally and emotionally killing Piggy.
Another way you could look at this is to imagine that Ralph is represented by the mother pig, as he was the nurturing character who oversaw the needs of the entire group. By the end of the book, this job has driven him almost insane. If you watch a few mothers in society, they do indeed juggle so much that they almost literally go insane.
A final way is to look at the actual death of Simon who could have saved the boys. If Simon is represented by that mother pig and mothers know best, then the boys lost the opportunity to hear the messages that could have saved them. He was indeed in the process of telling the boys about the fact that there was no beast and they couldn't hear the message because Simon's life was so swiftly removed from him.
I think Piggy's your best bet, but these are just a couple of other perspectives to consider.