The boys have formed themselves into a group and begin their ritual dance, which is their way of warding off fear. Instead of building shelters to protect themselves from rain as they ought to have done, Jack's tribe avoids confronting the dangers of life on the island by performing its own primitive rituals. Only this time Ralph and Piggy, those supposed paragons of reason, join in the fun, acting like blood-thirsty savages as they chant "Kill the beast!" and "Cut his throat!" over and over again.
As there's no actual real-life pig around, someone has to play the part of the beast in the dancing ritual. That role falls to Roger, which is appropriate given that he often behaves like an animal anyway. He pretends to be a pig at the middle of the circle while the other boys surround him, still chanting and screaming at the top of their lungs.
Not long into the ritual, Simon emerges from the forest to tell the other boys that the so-called beast is actually nothing more than a dead parachutist. But everyone's so fired up by the dance, so whipped up into a blood-crazed frenzy that they immediately turn on Simon, thinking he's the beast. And so they brutally slay him. The boys have now crossed the moral Rubicon; there's to be no turning back. From now on, they will descend further and further into outright savagery.