He was probably working for a cabal on the Senior Council Langtrey, La Fortier and Petrovich. Three can keep a secret if two of them are dead. This suggests Langtrey was behind the murders of the other two, Peabody was his cat’s paw against La Fortier, and got him to whammy Luccio. He betrayed Petrovich to the Reds. Du Morne was apprenticed to Petrovich former Warden a perfect choice if one was creating weapons.
Ah! But you say Langtrey was relieved when Peabody was exposed, but think how Harry did it, it would have been nigh on impossible for him to defend Peabody (Mai sided with Mouse) and politically it saved his public bacon, he hadn’t expected Morgan’s intervention, and that created a witness who had to be silenced, hence the manhunt. Peabody nearly escaped, except another intervention by Morgan ended him, and it was the plan all along.
The expulsion plan now is to keep Harry away from the White Council, maybe, because Arthur Langtrey is afraid Harry is going to put everything together leading to his exposure as.......Cowl.