It took this from Xenon in Yahoo answers and I agree with it. The text was in spanish so i'll try to translate it to english
In the last chapter of the first season, RJ's ex-girlfriend(Harker Rosalind), describes him as a good person (insists precisely on the fact that he is a very good person, and that allows him to see into people's inside) so Red John, or the character who represents, also appears to the viewer as a good person. The ex-girlfriend also describe him as high and Caucasian. This matches with Rigsby.
Another suspicious detail is that Rigsby and Harker Rosalind didn't appear together in the episode (so she couldn't identify him), and many other aspects, the conclusion is quite simple.
To give some examples, Rigsby shows great concern for the content of their personal file, which was read by Cho, and he demandded him what did it say about him.
On the other hand, Rigsby shows a strange trauma to women, a sort of inability to communicate with the beloved, or to confess his feelings, to Grace Van Pelt continuously. Apparently, it's a trauma that goes back to childhood, and the causes and consequences are, ineffably, behind the chain of murders starring Red John.
RJ is always aware of the steps of the group of Lisbon and Jane, something that could only happen if RJ is a real insider. So he could always go ahead of them.
Moreover, in the chapter on the hypnotists, Rigsby says at one point that, killing when you are drunk "could happen to anyone." What's more, in that same episode, when hypnotized, he reveals his true self violent, attacking several people, and being about to throw Patrick Jane from a rooftop when Jane had said minutes ago that "a hypnotized saint is still a saint, "that means that if he was a good person he would have behaved as one, no matter how hypnotized he was.
I hope the translation is good enough
