In this scene the degeneration of the boys into animalistic savagery is depicted. Their behavior mirrors a savage raid upon an enemy village: They not only capture their victim, but Jack violates the sow. For, as he slowly removes his spear from the female pig, the reader cannot miss the phallic symbolism. Sensing the eroticism of the moment, the other boys become "hysterical," and desecrate the pig further by decapitating it.
One critic, Claire Rosenfield, sees Freudian overtones in the characterization of Ralph, Piggy, and Jack. This scene in which the boys kill the pig clearly has Freudian touches of the fulfillment of desires of the uncontrolled Id, instinctive and base energy.
Jack and company hunt the pig; when its captured, Roger slowly drives his spear into the pig's anus.
The boys regress when there's no authority figure to maintain a code of ethics; however, the author states that "the moral is that the shape of a society must depend on the ethical nature of the individual and not on any political system." In other words, what happened didn't need to happen if the boys had individually acted ethically.
Much of what they do is necessary in order to kill a pig. They tried individual hunting early in their stay but did not succeed. Jack quickly learns where to find pigs, how to sneak up on them, and where their favorite places are. The first pig they kill is done by using a large circle of boys that slowly close in on the pig until they are tight enough to make the kill. The pig had nowhere to go (something we see later in the novel also with the hunting of Ralph). Obviously the chanting and dancing after the kill certainly isn't necessary, but it would sure be fun for a group of 12 year old boys.
With the next kill, Jack decides to sacrifice the head of the pig to the beast in order to keep the beast away from his tribemates. Again, we know that it's not necessary because we're privy to extra knowledge about the beast. Do the kids think it's necessary? Maybe....but again, necessary or not it would sure be fun.
Remember that these boys are away from any adult supervision and rules. Most of what they do on this island would be frowned upon and/or punished by authority figures at home. So even if dancing and chanting and sacrificing isn't necessary, the boys would do these things simply because they can.
Jack and his tribe kill the pig while chanting, "Kill the pig! Cut her throat! Spill her blood!" This is a very ritualistic killing. The chanting is not a necessary part of the hunt. While the boys should be spending their time maintaining the signal fire, they get caught up in the savage hunting process.