The premise of "The Black Cat," as in several of Poe's stories, hinges on one underlying question: Is our narrator sane? The unnamed man awaiting his death sentence tells us a story of "mere household events" as if he's going to rattle off a anecdote about something that happened at dinner. He then proceeds to mentally unhinge right before our eyes, yet throughout the story his rational tone always suggests the semblance of sanity.
The ultimate object of our narrator's undoing? His favorite pet, a black cat who dotingly follows him around, and whose eye he cuts out in a moment of drunken fury. The narrator tries to reason with us about the incident, dismissing it as some Freudian impulse: "Who has not, a hundred times, found himself committing a vile or silly action, for no other reason than because he knows he should not?" And, he has us a going for a moment (who hasn't done the bad-just-because thing?); that is, until "the spirit of perverseness" consumes him, and he unleashes his bloody fury on his unsuspecting wife.
In the end, the narrator's own guilt is the cause of his demise, and we're left contemplating his motives: Was he a sane man gone maniacal, or a maniac incredibly well-versed in seeming sane?