I think Bob Kirkland is not Red John. And Volker is not too.
The Scenarist trying to shock us. We dont know Red John's face.
I mean Red John is not a person of we know. For example: Jane or Kristina or Van Pelt or Bob Kirkland or people like these.
Red John is not a regular guy or killer. Jane is not a regula guy too.
They are different. They have some good skills. They are smart.
And They have a good sense of humor.
I know this is crazy maybe...

But my first guess: Red John is Jane's brother or half-brother.
I have strong reasons.. But will not explain now.

And my second guess:( this is weak guess) Red John is Jane'S father.
Remember the episode of his father. Jane cant accept his father doing.
And Jane going away. Maybe his wife bring him away.
So Jane'S father was very angry. And he took the revenge.
Jane's father wants back him(jane). Like old days.
Remember the 4. season final...
Red John is looking for a comrade. Remember the his words:

What if you really did have
a change of heart?

What if you and I were friends?

Imagine the life we could lead.

I dont know maybe these are compo conspiracy theories :)

Could be everythings... But I love The Mentalist.
The best movie ı've ever seen.

Have a nice life...

The man from the last episode bob homeland security guy I don't think that's red john I did but that's not a big enough twist for this show I think he works with red john very close to him like his number 2 honestly I have no clue who red john is I think it's somone we know though because obvioulsy this show loves big twist it's wierd how I care for these chartcters when I though jane was going to kill lesbon I was freaking out lol.

I think Red John is Robert Kirkland - you only see him twice in the show. He appears for the first time in Red Dawn. He is a friend of FBI's Californian chief Alexa Shultz. In If It Bleeds, It Leads, Bob Kirkland identifies himself to Agent Lisbon as a agent for Homeland Security and at the end of the episode he recognizes Jane even though he doesn't know him... 
Let's see on the next episodes but he looks very suspicious and creepy!

My guess? CBI Ron. Why?

In the last episode (season 5 ep 7) in the end the Bob guy says to Patrick Jane something like, you dont know me but i know you!, then when they go away, right before the episode ends, who is on focus? yeah... CBI Ron.

CBI Ron is always appearing. Always near Patrick..

And please check episode season 1 episode 16.. When Grace Van Pelt's new boyfriend is chasing van pelt and patrick jane in the parking lot with a gun, theres someone that runs away in his car. Who is it? CBI Ron!

A CBI Agent that watches some criminal with a gun trying to kill two CBI workers, and simply goes away without doing anything? That's really suspiscious for me.. I mean, he could not be Red John, but in some way he must be an accomplice or something like that. He is not a good person.. And we never heard his voice I guess..

Because why would the director of the series put a secundary actor like CBI Ron so many times during the 5 seasons? 

Rate my theory guys :D