I think, that Red John is Bob Kirkland's TWIN brother. They said about him in the series for a reason. And even if Bob Kirkland said that his twin brother was killed by Red John, I think it is not true. There is a possibility that Red John was an old man, started to kill people long time ago (evidence is "the face" on the building in one of the episodes). If that old man died, Bob Kirkland's twin brother could continue the work of original Red John.
I am betting for Evil Twin!!!

P.S. I think Bob Kirkland thought about it, that is why he was asking everyone "Do you recognize me? Have you seen me before?" hoping, that victims would mistake him for his twin!