Why does Simon’s role as a visionary make him an outcast in the group? What other visionaries have been outcasts in their societies? Why?
Simon is popularly interpreted as a Christ figure. Christ died at the hands of his people while trying to warn them of human sin, while Simon dies at the hands of his peers on the island while trying to warn them of the inherent evil of mankind - what they should really fear on the island. This makes him an outcast for a number of reasons, one being that at this point in the book, the boys are so wrapped up in their fear of the unknown that they cannot see past their mob mentality. Simon doesn't have a fear of the unknown because he has discovered what the fear should be of.
Visionaries tend to have unpopular vies and ideas. Human beings can be quite resistant to change or anything that "goes against the flow". In a book that is largely about mob rule and what happens when their are no rules to follow, Simon's radical suggestions are not met well.
Other visionaries include Socrates--who ended up dying for his beliefs. Gallelio, Martin Luther King, Jr., Abraham Lincoln, to name a few. Most visionaries tend to be outcasts and un-liked until their ideas become more popular.