I think that Brett Patridge was supposed to be Red John at the beginning of the series, but then the writers introduced a more interesting character in the second season: Bret Stiles.
Just think about it, he has been the most recurrent character of all the seven suspects and he has always been presented as a mysterious man with a great power to influence people (specially woman, like Van Pelt), he has shown a lot of sympathy for Jane and also, Jane has never been able to read him the way he reads other people.
I noticed that when Haffner gave Lisbon the information about Stiles whereabouts, he already knew that Jane was looking for him. So, basically he wanted to be found.
Also, I think that Haffner has been an errand boy for Bret Stiles, acting like Roy Tagliafierro and meeting with many of the people that have been identified as friends of Red John.
Many people don't believe in this because they cling to the clues related to the physical appearance and the age of Red John. I think that the first murder of Red John, and the birth of the happy face happened with Haffner as a disciple of Bret Stiles.
Finally, I want to add something else. Since the beginning of the series, Red John has not shown any rational criteria to select his victims, but if we think of Bret Stiles being him, we could also associate the murders with reasons related to religion and weird beliefs (like thinking that someone is inmoral and doesn't deserve to live). Also it would explain the extreme loyalty of Red John friends.
I just write a summary of some of theories posted here, adding some of my own ideas. Please let me know your opinions!