I believe Red John is a group of people.  That makes them more illusive.  There of course is a leader and that leader is in question, but many are in this group.  this makes sense to me because the people that help "Red John" are often killers or people hoping to rise in power.  Some of the greatest movies and television shows are based on secret groups willing to do evil things to rise to power.

Just a guess, but this seems the only thing that makes all of it fit.  I do not think it is Jane.  Nor kistina frye or grace.  

Thanks and good hunting us.