The murderer in this chilling story, that actually turns out to be rather ridiculous by the end, is the ourang-outang that belongs to the sailor that appears in the apartment of Monsieur Dupin in response to the advert that he placed in the newspaper. This creature committed the murder because of the way that it imitated the actions of the humans around him. Having spied its sailor/owner shaving in front of a mirror, the sailor one day sees him mimicing these very actions whilst wielding a sharp razor in his hands. When the creature escapes, the sailor is terrified to see that he grabs Madama L'Espanaye by the hair and imitates shaving her face with the razor. This play acting though takes a more sinister turn, as the following quote demonstrates:
The screams and struggles of the old lady... had the effect of changing the probably pacific purposes of the Ourang-Outang into those of wrath.
It is therefore the screams of the lady that cause the creature to move into a murderous rage and to commit the crimes that baffled the police to such a great extent.