I believe that Red John is not just one person, but a consortium of individuals that operate under the name Red John. This consortium might include Kristina Frye (who magically disappeared, apparently after being kidknapped by Red John, but has yet to turn up at all in the remainder of the series), Gale Bertram (who used the phrase "Tyger, Tyger" during a scene, apparently in reference to a phrase that only PJ ever heard), Todd Johnson (the serial cop killer who also said "Tyger, Tyger" to PJ), and Timothy Carter (who was killed by PJ during the season 3 finale because of a conversation he was having with Lisbon; he also dropped hints that he was Red John) to name a few. Maybe the reason so many plausible Red John suspects have died withouth his reign of terror ending is because he is not just one person after all.