Kemmler was the last Warden but one before Harry, it is indeed in A WOJ offered when Jim was asked about the Warden before Harry, and wouldn’t say, so it obviously plays into a significant plot point, and may have been someone we know or heard about. One of the deceased Senior Council perhaps? Obviously they hadn’t been to the island for some time.
My thought was that Warden was put in place to prevent Kemmler returning and reclaiming the wardenship, and really nothing more, but his role was enough to mark him for death.
I think it was LaFortier, whose murder in Turncoat is unexplained as regards motive, but what if it was a play for the wardenship, but before the Black Council could get their Warden in place Harry does a Sanctum Invocation. That’s the motive for his murder, to free up Demonreach and access to its prisoners. The mind control by Peabody was Plan A, but it wasn’t working or working fast enough. Murder was Plan B but then everything went all Dresden. La Fortier had interests in other parts of the world, once Warden he probably ignored Demonreach.
It also explains the interest in the Black Council with Chicago, Harry had settled right next to Demonreach, so his early cases were them aiming various people at Harry to get rid of him, Dead Beat was to tap into Demonreach’s ley line to power the Dark Hallow, Cold Days and Battle Ground had Nemesis making direct plays to break in.
The Black Council were never really interested in Harry, they were interested in Demonreach, looking to get one of their own as Warden and then a powerful rogue young Wizard moved to nearby Chicago, and ends up going to Demonreach in Small Favour. That may have panicked the Black Council, and why they set the captain of Wardens on him, not just because he wasn’t subject to Peabody’s influence.
The Black Council must really, really hate Harry, time and time again he buggers up their plans by accident.