What threat does the Beastie make to Simon?

The Beast at this point is manifested in the Pig's head on the stick. It is Simon's inner darkness. He threatens that all his friends will kill Simon. He says "they will do you" He says that Simon is not wanted and he is being a "silly little boy."

Simon, being Golding's Christ figure, purges the Beast but is killed in a way that the Beast alluded to.