In addition to the theory 7021 (http://www.whoisredjohn.com/See-a-theory-Red-John/7021), I can also add this:

The writing seen on the wall in 1.11, could be the beginning of "He is Mashburn", even if it is supposed to be "man/mar", Jared could started forming the top of an "s" which trailed downwards when he died.

Also, he has great ability in understanding people and defines himself as a person who "hates being ignored, cunning, ruthless, hates to lose", exactly like Red John.

In addition to that, Red John's first crime was in 1998. Around 1997, Walter founded Mashburn Avionics, a company for electronic components and weapons, which he sold and became very wealthy, rich.

Jane also suspected him in one of his RJ suspects list, in episode 5.9 (Black Cherry).