The name of the narrator's first cat, Pluto, is also the name of the Roman god associated with death; he is the equivalent of the Greek god, Hades, the Lord of the Underworld. Pluto is, then, heavily associated with darkness and death. Although the narrator initially has a wonderful and loving relationship with his cat, Pluto, that relationship degenerates as the narrator becomes an alcoholic (succumbing to "the Fiend Intemperance," he calls it) and turns violent and cruel. One night, when the narrator comes home drunk, he antagonizes the cat, who bites him, and he responds by cutting out the cat's eye. The narrator becomes more and more "perverse" in spirit, eventually murdering the cat by hanging it by the neck.
The narrator's abuse of Pluto begins a chain of events that leads to his own demise (after murdering his wife). In this way, Pluto seems connected to death and darkness, in part because of its color, but also because the cat's existence launches the narrator on a path consisting of death and darkness. The narrator's own disease, alcoholism, changes him, turning his soul black with sin. Though the cat itself is not evil, he seems to symbolize the influence of evil that impacts the narrator and turns him into a monster.