Lot of people think Red John is Patrick Jane with double personality. I think is more plausible the idea that he has a twin brother(same smart as Patrick but with an evil side) of course he is not aware of his twin brother, only RJ knows it.
For example if you see when patrick jane make his smiley is quite similar to red john blood smiley, what a coincidence.

Would also explain why RJ save Patrick in 2x23, and he never try to kill him.

At the end of first season, when he kill the cop, before he died, the cop said: red john, jane, shhh (like a secret)

Or when he interview Rebecca, and she said you and red john are so similar.