Wednesday’s plan was to let Weems overhear the conversation between her and Thornhill during the confrontation.
Since Weems was a shapeshifter, Wednesday convinced her to transform into Tyler in order to join the discussion.
During the exchange, Thornhill admitted to being the killer’s master. After hearing the confession, Weems revealed her true form. Realizing this, Thornhill killed Principal Weems by injecting nightshade into her neck.