In Chapter 8 of "Lord of the Flies", is Simon deluded and ill, or has he alone grasped the nature of what is going on?
Consider the Lord of the Flies' words "I'm part of you....I'm the reason why it's no go."
Simon is probably in the middle of an epileptic fit or its aftermath. During these times, reality is distorted and confusing for the person suffering the attack. His vision seems to be a confirmation of his earlier thoughts about the beast being part of "us". Simon has been consciously pondering the nature and existence of the beast and now, as he either goes into or comes out of an epileptic seizure, his mind focuses on his examination of the beast and seems to conjure up this conversation. The vision does reveal some truth about the beast and the nature of the boys but Simon will need to confirm it with a trip to see the actual "beast" and discover it's a simply a dead parachutist.
In the end, Simon has a pretty good grasp on what the beast really is. He has a first hand witness of what the nature of the boys is allowing them to do to each other. He alone will be privvy to the knowledge of the beast on the mountain, something that will ironically lead to his own death at the hands of the "beast". The conversation he has with the LOTF shows how great his understanding of the situation really is. Simon knows that his nature and characteristics make it difficult for him to convince the other boys about the real beast, and since he won't be able to convince them to stop, there will be no way to stop their actions.