I'm down with the RJ=Rigsby idea. He did show many strange things about his personality in the show. It always stuck me weird that a big dude like that let so many people make fun of him. What did Rigsby mean when he threatened FBI O'laughlin? We assume Rigsby is trying to protect Grace, yet the threat is a little more... cryptic... than what one might expect. Just sayin. 

Well, my thoughts are that Jane/jisbon has yet to decode the blake society list, and thus I am in favor of a Rigsby slasher ending to this lost television show. 
Anyone find it bizzare that Rigsby instantly knew where Jane buried his wife. The "Red John" S6 has Rigsby in the back of the truck stating clearly and without hesitation the exact name of the cemetary where Jane buried his wife. This is one strange thing among many strange things Rigsby does while in the back of the truck... calmly sitting the back of the truck. 
The only thing that makes me think that RZigsby is not RJ is the the blind lady says that RJ is under 6 foot tall. Rigsby is clearly over 6 feet. However, I do understand that RJ had many helpers. 
Oh Wait... Lorilie Martins also hits Rigsby over the head when she is going to kill her victim. Would Martins hit her beloved RJ over the head??? Methinks, not!
Aside from those two problems, Rigsby could easily be RJ.  

One thing we often forget it WHY Red john was called Red John?
Because he or she only killed Red Haired women early on in his serial killer career.
So i wish the show would explain why a serial killer broke his pattern?
Seemed like when Rigsby met Grace the red haired killings stopped.
But seriously tho, Rj went from a strict by the rules serial killer using black tape n all to just killing anyone any which way.
Like there was more than 1 red john!
And why all the sudden at the end Red John forgot how to slit necks?
Brett Partridge and Kira Tinsley? If Rj killed them right then Jane would of never found Macalister.
It takes a second to slit a neck too so no excuses!

The show needs to let us know if Red John was really a serial killer or just the Ba leader trying to cover up things.
Bc near the end Rj was not a serial killer at all, no smileys no black tape or specific victims, just random killings of people in his way.

These FBI advertisements, also known as The Mentalist, are slowly melting my brain. The levels of propaganda on television are staggeringly obvious these days, and I remain troubled by this fact only because I know the stupid mob takes this shit serious, or at least lets the idiot box form opinions about big government being "necessary". 
Why do we have to endure the FBI when CBI, a fictional agency of legal plunder, worked just fine? I mean, why does Cho and the other morons act so serious all the time? And, are we really supposed to find the stench of affirmative action interesting because it is carried on the shoulders of a once great, but entirely dead, series? Finally, does anyone really buy into the jisbon love affair as a possible hook to save this pathetic show/advertisement?
I think, and I am being serious, that I might enjoy this show more now if I were a jingoistic mental defective.

Rigsby is RJ  

Great series, but the last seasons were awful. FBI? Can't suspend disbelief on that.
If I were to pick a number one Red John candidate, then it would have to be Rigsby. His father was a biker dirt bag, he fell in love with the redhead, he says things that are out of character, and he is, like someone else said, a classically trained actor. There could be no better choice! 
If one were to watch the episode where he is on an island with the cbi team solving a murder, then one could see how Rigsby could be a great Red John.