Poirot is always on the lookout for details that are out of place. Nobody at Styles Court likes Alfred, but Evelyn’s passionate hatred for him still seems out of place, especially because she already disliked him so much
before the murder. Of course, Poirot doesn’t necessarily imply that Evelyn herself is the murderer simply because she has the capacity to hold onto such vehement hatred, but he
does suggest that the general nature of her rage is a bit fishy.