For me the Red John Killer is Patrick Jane's unconscious, that for me it's different from a second personality but it's the real and hidden jane's personality, making it something new and not another "fight club". (Sorry for my english but it's not my first language)
As I heard in the 5x08 episode, Patrick and Red John met I felt this impulse to investigate who Red John is, so I went online and look for fan theories or usefull recaps, when I happened to find this page my first suspect was JJ Laroche because he's manipulative (he bahaved that way with Rigsby, Vanpelt and Lisbon) we also know he has a dark secret that must be terrible, he has a soft voice, and as Lorelei says they didn't become lifelong friends as they met while with other suspects Jane has a much better relationship. But here I read another user's post where he named a book by C. G. Jung "The Red Book" so I got information about this: it was released for the first time in 2001 (and published in 2009) as Jung's heirs refused to make it public, the book is about an experiment he did, a voluntary confrontation with the unconscious, through active imagination tecniques (something connected with meditation or I suppose self hipnosys). This could be the key to understand the obsession with the red color, and as a second reference in the last episode of the 4th season Jane's reading a book by C. G. Jung, I must add that the release of the book to academics in 2001 matches with the time should take to develop a plot and film a pilot episode in 2007/2008.
After reading this I begun to connect the odds, for example no RJ follower harms Patrick, the only person Patrick kills is a RJ copycat and the only one who claimed to be RJ (his wife was convinced to be married to red john) so he was the only copycat Patrick's conscious would accept to kill without a breakdown, than Simon Baker has a strange eye's cut so when he smiles the RJ logo seems a "ghost like" caricature of Patrick. More clues are given by Lorelei che calls Patrick "my love" in interrogation room, and when Patrick picked her up outside the prison she look into the car approach the door with an happy attitude and only after Patrick speaks she shows delusion, this means she was not able to tell he was not RJ just by the look, than when she only gets angry when she discovers Patrick made every one to belive she kidnapped him but when he tells her RJ raped and killed her sister she just wants to run away from Patrick.