Esteemed Mod-
Is in not likely that in Riddle World there are some narrators who always lie?
Even if we assume the narrator is completely objective and truthful, what evidence do we have against the maid? I would argue that her lie points to the state of her mind affected by the tragic news and not to habitual lying. I would guess that upon investigation her true alibi will be discovered. On the other hand, the butler and the cook displayed shocking coolness under the circumstances. The callousness could indicate premeditation or the fact that they are habitual liars.
Everyone knows that the butler always did it, so it makes sense that he would be unaffected by the murder- much like a homicide detective is completely desensitized. Thus, the cook is the most likely culprit.