Not really much of a theory, but I've been thinking.

Even though I hope this isn't the case, it might be possible that red john is a psychic.

'He is mar' could be 'He is magic' as well. It could be an unfinished G. If not, it probably is 'he is many', because I don't see how the handwriting would indicate a proper R or a D, in 'he is married' or 'he is mad'.

Just chose Haffner because I like him for being red john, although I think the evidence we have is way too ambiguous for me to be sure it's him or not.