My theory is Kirkland. In the end of the last episode, he kills Lennon. But what's most important is the timing: it was after Lennon woke up from his coma and Kirkland realized that he remembered him. Lennon was the one person that could lead Jane to Red John. Coincidence? I think not.
Kirkland was calm in administering the deadly poison--very stealthy, very cool, very collected. And when Jane got there, his manner did not change.
The words Kirkland spoke struck me: "I'm so sorry. He didn't say a word." Was he sorry? Maybe. But the second sentence finalized Lennon's death and the link Jane saw to Red John.
But the most important clues come right after. With three fingers, Lennon strokes Jane's shoulder as a sign of apology and friendship. But do most people stroke in a semi-circular arc? I see a connection to the distinct three-fingered smiley faces Red John paints after killing a victim.
As he walks away, Kirkland's face is blank, cold, hard, emotionless. It's like he wipes the memory of the death clean off and moves on like nothing happened. It's the face of a cold killer.
Earlier in the episode, Patrick's inside man (the nurse) comments how Kirkland and his partner feel like monsters in human skin. Could it be that they really are? That Kirkland is that monster--that monster known as Red John?
Then, notice how Kirkland's ties throughout the episode are a deep red: the color of dried blood.
And he dresses neatly, and he is well-groomed. Notice how all his killings are neat and have the same attention to detail as Kirkland does in his dress.
And it is that detail and his perfect history of not getting caught, his power and influence through TV shows and more, and his connections within the city would rule out a man in his twenties, and likely any man over 60. Kirkland has a touch of gray in his sideburns and slightly-graying hair. I'd say he's 40 or 45, putting him in the perfect age range for our Red John.
From this overwhelming evidence, we may have gotten a glimpse of our cold serial killer.
