I believe Red John isn't a single person I believe it's a network of killers. A network trained by a man called Red John but a long time ago nd now this Red John is dead. Kindve like the saw movie where his work goes on. Jane might have shook hands with Red John, so he thinks. Jane thinks it's a single person but the Red John network,having known this, probably are mentalist them selves. Trained to lie and not be discovered. It could be any of the suspects or maybe all. Red John is not a person it's a secret network.