I really hope we'll get an explanation for the writing on the wall. A lot of people seem to think it's "he is marked". I don't think so. It seems very strange that a dying man would write that on the wall instead of just 'tattoo' for example. And anyway, how would he have seen it? Was Red John killing him in a sleeveless top? lol
 
The only reasonable explanation for me would be "he is marshal" which is something like a sheriff if I'm not wrong? He was obviously wearing his uniform and because the guy didn't know his name, he didn't know what else to write..

I think it's much more believable than "he is marked". That wouldn't be a big help, people can be marked in different ways.