If we are to believe the narrator of “The Black Cat”—and that's a pretty big if—he used to be a kind, considerate human being with a sentimental attachment to animals. Back in the day, his house was a veritable menagerie, jam-packed with all kinds of pets that the narrator would happily feed and caress.

The narrator further goes on to tell us that this “peculiarity of character” of his remained as he made the transition from boyhood to adulthood. He positively reveled in the companionship of animals, whose “unselfish and self-sacrificing love” was vastly superior to the “paltry friendship” and “gossamer fidelity” of men. In other words, the narrator preferred animals to humans.

Yet by the time we've reached the end of the story, the narrator has experienced a profound change—and not for the better. In a fit of rage, he kills what was once a beloved pet cat called Pluto. Later on, his brain further tormented by alcohol and mental illness, he kills his wife with an ax—but not before trying to kill another one-eyed cat he'd befriended.

By the time the police break down the narrator's walls and discover the corpse of his wife—not to mention the black cat he tried to kill—his descent into madness is complete.

In Edgar Allan Poe’s “The Black Cat,” the narrator experiences a descent into madness. The object of his obsession is a black cat named Pluto, one of his and his wife’s many pets.  A previously affectionate relationship between the narrator and the cat (“Pluto . . . was my favorite pet and playmate. I alone fed him, and he attended me wherever I went about the house”) begins to degrade with the former’s alcoholism.  As Poe’s narrator describes it,

“Our friendship lasted, in this manner, for several years, during which my general temperament and character—through the instrumentality of the Fiend Intemperance—had (I blush to confess it) experienced a radical alteration for the worse. I grew, day by day, more moody, more irritable, more regardless of the feelings of others.”

The narrator’s psychological state – in effect, his transformation from one-time normal human being with an abiding love of all animals into an alcohol-infused sadist.

It is in this reduced state that the narrator, increasingly angered by the cat’s avoidance of him, gouges out one of the cat’s eyes.  As the cat recovers, it remains frightened of the narrator, whose disposition continues its downward slide:

“And then came, as if to my final and irrevocable overthrow, the spirit of PERVERSENESS. Of this spirit philosophy takes no account. Yet I am not more sure that my soul lives, than I am that perverseness is one of the primitive impulses of the human heart—one of the indivisible primary faculties, or sentiments, which give direction to the character of Man.”

When the narrator hangs the cat from the branch of a tree, he remains entirely cognizant of the immorality of his actions: “I knew that in so doing I was committing a sin—a deadly sin that would so jeopardize my immortal soul as to place it—if such a thing were possible—even beyond the reach of the infinite mercy of the Most Merciful and Most Terrible God.”

With the passage of time, and his house having burned down, the narrator and his wife resume a seemingly normal existence.  The appearance of another large black cat, however, this one with a white breast, presages another deterioration into madness.  The white fur on the cat’s chest begins to take the form of a noose, and the narrator begins to loathe this cat as much as the one he earlier mutilated and killed:

The narrator changes from a quiet, compassionate child to a madman, prone to uncontrollable fits of rage and--in his own words--perversion in adulthood. 

He tells us that even from infancy, he was "noted for the docility and humanity of [his] disposition," being so compassionate that his peers mocked him. He particularly loved pets, and his parents got him a variety of them growing up, and this is an affection that followed him into adulthood. Apparently, the pets felt the same about him because after he married, he and his wife collected pets, among them a black cat named Pluto. 

Over time, he changes. He blames this on alcohol. He becomes "more moody, more irritable, more regardless of the feelings of others." He turns to insulting his wife then to beating her. Eventually, he turns his "fiendish malevolence" upon Pluto, cutting one of its eyes from the socket one night. The cat, when it heals, runs from him (of course!), and at this point, the narrator admits that in a fit of uttter perverseness, he hangs the cat by the neck from a tree. 

Eventually, the cat is replaced by an eerily similar one which gets on his nerves, too. When he tries to kill it with an ax and his wife stops him, so he turns on her and kills her with it. Once he disposes of the body, he is finally able to sleep in peace (after murdering his wife!), simply because the cat is no longer following him around. 

He has thus progressed from a gentle child to a violent psychopath.