The LaFortier theory
LaFortier was murdered after the Black Council found out that he was secretly the absentee Warden of Demonreach, as this meant one of their number could assume the role. Harry however blundered in before their candidate (Peabody) could discretely beat the vacillating White Council to the punch. Peabody was an expert on summoning and binding (his book on the Erl King) making him the ideal candidate. LaFortier appears to have had a similar background, given the description of his rooms. The motive ascribed by Eb that it was to make way for Christos as a Black Council nominee doesn’t stand up following Christos actions in Battle Ground. More Ink I think, playing on the fact Eb and Christos are chalk and cheese.
The identity of the Warden was closely held amongst the Senior Council, Peabody was obviously looking for clues using his ink. He may have had a previous misfire incorrectly identifying Simon Pietrovich, his murder by the Red Court may have been what Cowl was after from the Red Court in Grave Peril, and he and Peabody supplied the information to get past his defences.