I don't really have a good idea on who it is -  but I am pretty sure who it isn't - Bob Kirkland.

There is a timeline issue as to whether Lorelei could know that Bob and Patrick have met as she was incarcerated at the time.

There is the manner in which he kills Jason Lennon - no trademark killing here.

But for me the biggie is Jason knows RJ (hence the reason he has to die) and he doesn't know Bob Kirkland. Therefore Bob is not Red John.

If it has to be one of the seven, I have to go with Brett. But  I wouldn't be surprised if it were not one of the seven. I think Patrick set a trap with the board and the seven names. Obviously, RJ knows the names because he broke into the attic and saw the results. And Patrick knows he did.