I was thinking about the clue " He is Mar " (assuming BH does not make random clues without explanation) and a couple of stuff came to mind. Firstly,when he was writing that on the wall maybe his intention was to write n rather than r, but died before he could do that. That means he wanted to write "He is Many" or probably Red John's name. 

I checked the dictionary for possible words starting with "Mar" and i could not find anything that would point out to this clue,but there was a word with initials M.A.R with a definition "Master of Arts in Religion". Is Brett Stiles a master in the art of religion? He is the head of a cult so it's natural to assume that he is a master of religion. RJ also has ties with visualize so that must mean something.