I already said how I think it's Haffner for so many reasons and for his feelings towards Lisbon leading to not killing her.
What I want to say to you most is, BOB KIRKLAND and PARTRIDGE are DEAD. Stop making theories about them.
Another thing is, now I'm getting carried off by this Michael Kirkland :P As someone said there is no way we've seen him on the screen, but maybe we did ? IT'S a TWIN, maybe we were sometimes looking at Michael thinking it was Bob ? Maybe in the hospital scene it was Michael, maybe he just uses his brothers position to ge into places like the closed wing of the hospital ? 
Haffner VS Michael:
Haffner just doesn't have the thing, the looks. Kirkland twins (supposing they look the same) have it. They look scary, freaky face, very suitable for serial murderer.
I can imagine now in the last episode the flashbacks from all the scenes where we looked at Bob but it was actually Michael (RJ), where Michael dissappears and Bob shows up.