I've always entertained the thought that Lafortier figured out the ink—not that that was the vector of transmission for Peabody's influence, but that there was influence, and had begun narrowing down the suspect pool. He managed to fight off most of the control once he recognized it, but didn't realize how subtle it was, and his natural paranoia and distrust of warlocks (such as Harry Dresden) was amplified. He had it down to a super short list, but didn't bring anyone else on, because he couldn't be sure who he could trust. He was too close, so Peabody killed him. He didn't know precisely that it was Peabody, but he was down to, say, the last four or five suspects, and it would've been a matter of days.
That's how it played out in my head anyway.