The narrator of "The Black Cat" confesses to being plagued by mood swings. He blames this on his bad drinking habit. These mood swings do not merely make the narrator grouchy but incite him to both verbal and physical abuse of his wife and animals. He describes his drinking habit as a "fiend" and he becomes as much, terrorizing those he claims to love more than anything.
The black cat Pluto is the one creature who avoids his wrath for a while. The narrator loves Pluto dearly, describing him as a beautiful animal who is devoted to him. However, once his mood changes for the worse, Pluto begins avoiding his master. This avoidance inspires rage in the narrator, who eventually murders the animal by hanging. The cat is not the only victim of the narrator's homicidal rage: later, the narrator also murders his wife when she attempts to protect a second black cat from his vengeance.
Though the reader is told that the narrator was gentle during childhood and that his current villainy is only from drink, there is some uncertainty regarding just how much alcohol is to blame for his moodiness and violence. For example, when the narrator describes the prelude to Pluto's hanging, he does not blame drink but a sense of delight in committing evil for evil's sake:
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. Who has not, a hundred times, found himself committing a vile or a silly action, for no other reason than because he knows he should not? Have we not a perpetual inclination, in the teeth of our best judgment, to violate that which is Law, merely because we understand it to be such?
From this passage alone, one has to wonder if the narrator is as naturally gentle as he originally claimed. He exhibits a similar perversity in the murder of his wife and his glee in being able to hide her corpse from the police when they make their initial sweep of the house. The strange mood swings might have been made more intense by excessive drinking, but the narrator might have always had these cruel impulses within him without being aware.