Greetings. I belive that Red John is not one person but in fact the organization.
First of all, do not mistake me for a conspiracy theorist but I like to think of Red John as he is the leader of powerful and well connected group ( i.e. Illuminati etc.).
This is due the fact he has lot of minions doing his dirty work, protecting his belives and identity at cost of their lives. He is very intelligent, like prof.Moriarty in Sherlock Holmes. Individual that holds FBI agent as one of his minions is no ordinary man.
That is why I believe RJ could be organization of many, led by few or even one which never got his hands dirty.
Sorry if my english was bad I am from Croatia and it's not my native language.