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.