What does Simon fear will happen if he tells the other boys that he knows what the true beast is?

The pig's head is a manifestation of Simon's inner struggle; it is a struggle against good and evil. Simon fears the boys will not believe him and, like the voice warns, hurt him for ruining their fun.

The pig's head is a manifestation of Simon's inner struggle; it is a struggle against good and evil. Simon fears the boys will not believe him and, like the voice warns, hurt him for ruining their fun,

"Dont you agree?" Said the lord of the flies...Aren't you just a silly little boy?

..."Fancy thinking the beast was something you could hunt and kill!...You already knew didnt you? Im the part of you? Close close closer! Im the reason why its no go? Why the horrible things are what they are?...So dont try it, my poor misguided boy, or else...We shall do you in. See? Jack, Roger, and Maurice and Robert, and piggy and Ralph"

Simon, of course, conquers whatever darkness lies within himself. He knows the risk but says, "what else is there to do". Like the Christ figure we all expect, Simon goes down the mountain to proclaim the good news and, mistaken for the beast, is worse than crucified.