I have watched in the past month all 122 episodes, which makes me a serious Mentaddict. =)

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-Spoiler for those who did not watched "Fire and Brimstone"-
At the end of the episode the 5 remaining suspects are separated into two groups. Those who have the tattoo(3) and those who do not(2).

Episode 123 is called "The Great Red Dragon" where the law enforcement mafia will be revealed. Then the three tattooed suspects will be discarted.

Episode 124, "Red John" one of the remaining two will be revealed as the serial killer.
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Ok, I believe that is the storyline of the series. Bret Stiles will be revealed as Red John "the mind" and Michael Kirkland "the body".

My guess: The law enforcement mafia has used Red John as a cover up for commiting crimes. The day Red John saves Patrick revealing the tyger tyger poem, he needs Jane´s help. Red John feeds him to help him uncover the mafia who are using his signature to commit crimes. Being Brett Partridge the ideal member to cover up the leads. Red John kills him first as a call to the mafia "I know who you are and I am after you". Reason why they really need to find him first. They need to shut him up before Red John reveals not all the murders where commited by himself nor his accomplice. Red John loves Patrick as a father-son love. He killed his wife and daughter not to punish him but to convert him into a killer, which he already has by killing Red John´s bait.
Michael Kirkland "the body" is Red John, or if you want to see it in another way, he is Red John´s main assasin. He fits in size and description to most leads given. Bret Stiles "the mind" is Red John. He fits all the cleverness to be always ahead of Patrick.

Well, in two week we will see, until then... =)