Chapter 9 of the Lord of the Flies is the chapter that takes the boys from just being boys to being killers. Simon discovers that the so called beast of the island, in really nothing more than the body of a dead airman. His parachute had become tangled and stuck in the trees. Simon cuts the lines of the parachute and allows the body to slip away. He realizes that he has to go and tell the other boys that there is no beast.
In Jack's camp, they are having a fire and a feast. Ralph and Piggy have come, to try to see if Ralph can see what is really going on and try to get control over the boys. They start to chant and dance around the fire. Ralph and Piggy join in slowly, and get caught up in the frenzy that is going on. Simon rushes in, to tell them there is no beast, and Jack and the other boys, in their frenzied state, think that Simon is the beast. Before Simon has a chance to say anything, the boys attack him.
"The beast struggled forward, broke the ring and fell over the steep edge of the rock to the sand by the water. At once the crowd surged after it, poured down the rock, leapt on to the beast, screamed, struck, bit, tore. There were no words and no movements, but the tearing of teeth and claws."
This is the moment when the boys have crossed a line and there is a question to whether they can ever come back from what they have done. Simon represents good, and they have just killed the good on the island. Even Ralph and Piggy had a part in the death of Simon. The boys saw Simon as the beast, when in reality the beast is inside of all them at that moment.