Just a theory but I think we can eliminate Bob Kirkland from the suspect list.
Why? Well first because it would be too obvious, i mean, Bruno Heller clearly wants us to see him as a suspect and it would be way too easy.
And the second thing is related to the "killing in the hospital" part. It doesn't fit RJ MO, I mean, killing like this is almost too soft for RJ (remember when he killed loreleil, it was supposed to be a "soft murder" because he liked her).
And here, Bob Kirkland just kills in the hospital, with people around and no blood, it does not look like Red John at all.

Let me know what you think about it!