Going on the theory that Red John is someone we saw in the first few seasons, and the writers hinted that he was hiding in plain sight, then it makes sense that CBI Ron is Red John. For starters, he's about the same size as RJ, he has insider info on the cases, and could have used his position to get to people like Rebecca, Renfrew and Wainright. Plus he's unassuming, he could looks like he could easily hide the fact that he's a mass-murdering sociopath.