The narrator explains his change in attitude toward Pluto as resulting from the "Fiend Intemperance." Intemperance is the nineteenth-century euphemism for alcoholism. We are given to believe that drinking alone is what changes him from a mild-mannered, affectionate man to one who is abusive to his wife and to his pets.

He claims that in spite of behaving cruelly to the other pets, he at first retained his affection for Pluto, but then, as his alcoholism became worse and the cat grew old and "peevish," he began to abuse him as well. The immediate trigger for the first sadistic act against Pluto is that when the narrator returns home drunk one evening, the cat seems to avoid him, and then when he seizes him, Pluto bites him. It's only a small wound—if a cat really wants to hurt someone, generally he'll use his claws, but Pluto does not do this—but the narrator then takes out his pen-knife and mutilates the animal, cutting one of his eyes out of the socket.

It is usual for commentators to regard the narrator of "The Black Cat" as an unreliable one. His tale is so bizarre that we have no way of knowing if his account of his former self as a mild, kindhearted person is true within its fictional context. Poe may be implying that the narrator has fantasized much of the action of the story or re-interpreted it to present himself in a less gruesome light, though if this were the case, one wonders why he would admit having done any of these criminal acts. However, the man tells us at the outset that his execution is imminent, so it makes sense that he would be truthfully unburdening his soul to his literary audience.

Is it plausible that alcoholism would change a mild-mannered animal-lover into a sadistic criminal ? That this can, and does, happen seems to be one of the main themes of Poe's story. "But my disease grew upon me," the man says, "—for what disease is like alcohol!—and at length even Pluto . . . began to experience the effects of my ill temper." This sentence expresses one of the most insightful thoughts in all of Poe's work—and perhaps in all of the literature of the nineteenth century dealing with addiction. Generally, in Poe's time, alcoholism was not thought of in clinical terms as it is today. It was considered chiefly a moral failing, though Poe recognizes that "disease" is the correct term for it. Poe himself had problems with alcohol and, probably drugs as well, throughout his brief life. Evidently, he knew what it meant for someone to be an "angry drunk," and some alcoholics do indeed become violent when under the influence. One has to believe, however, that in a case where such horrific violence is inflicted, the person must have had sadistic tendencies to begin with, which have remained dormant but are then awakened when his drinking problem worsens. This is the best explanation for the narrator's behavior and the tragic results of it.

The short answer? Alcohol. The narrator describes how his "general temperament and character -- through the instrumentality of the Fiend Intemperance -- had (I blush to confess it) experienced a radical alteration for the worse." Basically, he went from a loving husband and parent to many beloved pets to an angry drunk who would lash out at both his wife and his animals. He describes verbal as well as physical abuse on both.

Still, he kept from harming his favorite, Pluto, for a time, showing that Pluto's well-being was more important to the narrator than that of his own wife. Eventually, though, even Pluto is the victim of the narrator's violence, leading up to a terrible moment. 

The narrator begins suspecting that Pluto is avoiding him (only natural, considering his behavior). The thought of his beloved favorite avoiding his presence enrages the narrator and he grabs the cat forcefully. Pluto bites his hand, and the narrator responds by carving out Pluto's eye with a penknife and then hanging him, dead, from a tree. 

Clearly, the narrator's actions are totally outside the realm of reasonable reactions. Blowing Pluto's behavior out of proportion – or even inventing it entirely – causes him to murder his favorite creature. I think the blame lies squarely with the narrator and his terrible alcohol addiction.