The question is who committed the 2012 killing if it wasn't Rust? I'm with everyone else in thinking that he certainly won't be the one responsible simply because of how obvious it would be, but at the same time we're so far in and we're only getting to the character that they assume did it the first time around. With how well written this show is, I couldn't see them throwing out a character that hasn't been introduced yet to make him/her the killer, especially with the pace at which the show moves.
I'm going to say it goes unresolved or (screw it) Rust did it and confesses in the final episode as the culmination of an incredibly bleak and disheartening speech and cuts his own throat with the knife, having finally gotten over his inability to kill himself. Could definitely see the knife as a big old Chekov's gun. I don't know. Maybe it was actually Marty and the girl is Alexandra Daddario or some shit, considering that I doubt he ever actually gave the detectives her name and Rust wasn't aware of Marty's sidepiece's identity.
Edit: Nevermind, I'm throwing all my eggs in the Marty basket. Rust sleeps with his wife, Marty ends up divorced. Alone, angry, and drunk, he turns into a man beyond a certain age without a family. We've seen that he's prone to bouts of extreme and near-uncontrollable rage, he's "all fucked up", and even though he's "not a psycho", we all know he's full of shit. Snaps one day when he's drunk, kills a girl, does it up to look like the serial killer because he knows that everyone'll be looking at Rust (revenge!), especially when he gets the chance to play "stand by your partner" in the interview while still talking about what seems like underlying mental instability. Once again, full of shit.
Could just be a random girl. I can't remember if they've given any specifics about her, but it would make sense for him to snap on Daddario years later after seeing her again. Her acting run is done on the show according to IMDB, but they could still throw a morgue photo in no problem. Rust wouldn't do it because he's not a dynamic character. He knows who he is and what he does, and he's not going to go through that sort of drastic change into a murderer, let alone that kind of murderer, so late in life. He's just gonna keep on being bleak and lacking the willpower to kill himself. Gonna be exciting seeing how wrong I am.
Ugh, this show.
I've never felt the urge to curse the game itself over the teams in it, but fuck you Superbowl.