why does simon feel a "perilous neccesity to speak"? what does he think the beast is?

Simon does not like to publicly speak. He keenly feels the stress and the bad energy that the boys create. With all the talk and fear of the beast, Jack feels that he needs to reveal the truth. Simon knows that the only beast (evil/ fear) on the island lies within the boys themselves. They serve as their own antagonists. This info might have saved these boys but the meaning is too complex; it is lost on them.