The killing of Simon represents the final blow to Good in the battle of Good vs. Evil. Simon is a Christ symbol. He represents both clarity and truth. He wants to tell them that there is no beast. That was a man on the hill, not the beast.
As he climbs down to the beach, the kids are chanting, “Kill the beast! Cut his throat! Spill his blood!” (ch 9)They chant this over and over again, and eventually Ralph and Piggy join in. The two of them would rather be a part of a group than to be outside of the circle.
"They were glad to touch the brown backs of the fence that hemmed in the terror and made it governable." (ch 9)
Then as they destroyed Simon, "There were no words, and no movements but the tearing of teeth and claws."
The beast had finally come alive, but it was within the boys. It was not anything else on the island. Once they killed Simon, all that was good and full of truth was dead.