I think this answer is both literal and figurative and it has everything to do with the perception of evil in man's heart.
First, literally the Beast could not harm Simon because it was dead. It was just the illusion that all the boys had seen at different times of the dead man whose parachute got stuck in the trees and the wind man this "beast" appear to move. The horrible of this was the fact that here lay a dead man, in stages of decay. This means that there could have been a terrible stench, flies and other rodents may have been eating away at what they could reach. Literally, this scene could have horribly affected a child, although for Simon he had a message of great relief to bring to the rest of the boys.
Second, Simon is a figurative representation of the Christ-figure or goodness. Simon had never done anything to hurt anyone, he only suffered for it. In having this knowledge of a "victory" over the beast, or over evil, or of that which was harmless, Simon was ready to offer great relief to the rest of the crew from the fear that the idea of the beast imposed on all the boys. However, that message, that truth he wanted to share, was the very message that got him killed, therefore he died horribly. When Simon approached the kids with this great news, they were already in a trance getting ready to "Kill the beast!" Ironically, that which the kids hoped to destroy was their only hope of salvation from fear.
Golding does this to demonstrate to society that we must be careful to not destroy that which is good by being so overcome by evil. We must measure carefully what we allow to enter our perceptions.