When he regained consciousness after his fit, what knowledge did Simon feel compelled to reveal to the boys? Why did he feel this was critical to them?

Simon is the only boy who understands the nature of the Beast; he knows the beast lives within all the boys. So, it is fitting when Simon scrambles down the mountain to tell the boys the "good news" (The beast that they see is just a dead pilot), Simon is brutally killed. In trying to save the boys from themselves Simon is murdered. If you check out the last paragraph when Simon is killed (the language turns almost spiritual) Golding leaves us no doubt that Simon was the boys' Christ figure and he is sacrificed.