What desire do the boys feel as they dance and chant?

The dance in Chapter Nine is far more than a ritualistic offering. The boys move in circles, frenzied and crazed with bloodlust from their successful hunt. It is a savage scene; there's absolutely no "civilized" behavior left to discern as they've all become like wild animals themselves. Ralph and Piggy even join in by skirting the edges and dancing along.

The group then goes on to reenact the hunting of the pig once more, and the dance reaches an even higher pitch of frenzied energy, but when the boys see a figure creeping out from amongst the trees they are distracted and because of their disorientation fail to see it's not prey but rather Simon. They shout out in excitement; they believe Simon to be the beast, and they attack him. Simon is beaten, and the boys attempt to tear him a apart with their bare hands, all while he struggles to escape and get them to realize who and what he is. When he momentarily frees himself, he is able to run just a bit, but he stumbles and falls over the rocks. The boys, out of their minds and completely without rational thought fall upon him and murder him. This dance and scene are an example of how barbaric the group has actually become.