The darker side of human nature is clearly involved in "The Black Cat" as it involves sadistic actions and even murder. The narrator writes that he has been fond of animals,
There is something in the unselfish and self-sacrificing love of a brute which goes directly to the heart of him who has had frequent occasion to test the paltry friendship and gossamer fidelity of mere man.
Yet, when he begins to drink, the "fiend intemperance" causes a "radical alteration for the worse" in his nature. As the narrator allows his darker side to become active, he maltreats the animals. One night after he returns intoxicated, he fancies that the cat "avoided my presence." Seizing the cat he puts out its eye with a penknife! Here the narrator exhibits another gothic element: "privileged irrationality and passion over rationality" as stated in the enotes on Gothic elements. And, this irrationality is pivotal to the plot of this narrative.
This irrationality in the narrator becomes chaotic as "the spirit of perverseness" dominates and the narrator's actions become horrific. One morning "in cold blood" the narrator hangs the cat recounting that he "knew it had love me and had given me no reason of offense." On the night of his cruel action, the narrator's house burns. Returning to the house, he finds a bizarre "bas-relief" in a portion of wall where there was fresh plaster. There the cat with the noose about its neck had been imprisoned.
As he continues his debauchery, the narrator espies a black cat sitting on a cask of liquor. When he asks the owner of the establishment in which he has been drinking if he can purchase the cat, the owner says he knows nothing about this cat. So, the narrator takes it home, noticing that one eye is also missing in this cat. He feels an aversion for this cat, but avoids killing it as he fears the cat, as it inspires "horror and terror" in me. There is something preternatural about this cat, another common aspect of gothic tales.
As the narrator's irrationality increases, he imagines that the cat never left him alone, even putting itself upon his face. Finally,
Beneath the pressure of torments such as these, the feeble remnant of the good within me succumbed. Evil thoughts became my sole intimates, the darkest and most evil of thoughts.
One day as he descends his stairs, the cat gets underfoot, "nearly throwing [him] headlong." At this point, the narrator becomes incensed and grabs the cat, "uplifting an axe." But, when his wife stays his hand, "goaded by the interference into a rage more than demoniacal," the narrator puts the axe into her head and kills her. His chaotic actions done, the narrator decides to wall "it" up in the house in order to hide "the hideous murder accomplished." So, he walls in the corpse. Afterwards, he cannot locate the cat, yet he sleeps "soundly." Much like the insane narrator of "Tell-Tale Heart" the narrator, in his "mere frenzy of bravado," boasts of his walls when the police come.
When they again return, he takes them around. He tells the men he is glad to have "allayed" their suspicions, striking the walls hardily. As he does so, there is a scream,
utterly anomalous and inhuman--a howl, a wailing shriek, half of horror and half of triumph...from the throats of the damned in their agony and of the demons that exult in the damnation.
The police respond, tearing down the wall and the corpse falls. There on its head, sits the cat!