One theme of "The Black Cat" is that of transformation.
Poe's unreliable narrator undergoes both physical and psychological transformations throughout the narrative. From the beginning, this narrator exemplifies a changing personality. Even though he declares himself not mad, he mentions that he will relate what has happened, calling the bizarre incidents a "series of mere household events." In addition, shortly after declaring the events commonplace, the narrator expresses the hope that a "less excitable mind" than his will examine and explain what has happened.
Further in the story, the narrator admits that his mind has undergone "a radical alteration for the worse," and he has become "more moody, more irritable, more regardless of the feelings of others" because of the "fiend intemperance." He mistreats the rabbits, the monkey, and even the dog, but not Pluto, the cat. However, it is not long before Pluto also suffers mistreatment. When the cat inflicts a wound on his hand, the narrator feels as though a demon has taken his place, and he reacts by cutting one of the cat's eyes out.
As the story progresses, the narrator becomes more and more abusive until he raises an axe in order to kill the cat. But, because his wife arrests the blow he intends, he pulls his arm from her grip and "burie[s] the axe in her brain." Then, he sets about "deliberately" to dispose of the corpse. The narrator fails to understand that he has transformed into a "monster" himself.