Before the list my two suspects were Mashburn or Bertram. Mashburn was a suspect for the money/resources and the free time to kill people and Bertram was a suspect for his police connections. I am left with Bertram as a suspect.

Like many of you I was puzzled with the strange meeting Bertram had with Kirkland about trusting Teresa. However now it makes sense. Kirkland was trying to identify people in the tiger tiger society by pretending to be one of them (meeting with Reed Smith). So Robert Kirkland probably told Bertram Tyger Tyger but Bertram being Red John knew that he was not in the TT society. 
Also remember when Rebecca told Jane that RJ wanted him back on the case so he wanted Bosco dead. All supervisors that were between Bertram and Jane were either killed or moved away so Bertram /RJ could be closer to PJ.

My only problem with Bertram is the timing. How would someone in that kind of job would have the time to go on a killing spree? Also Jane had Bertram all this time under his nose and was not able to see through him.