What does the narrator say is the reason why he is abusive in Poe's "The Black Cat"?
At the beginning of the story, the narrator blames his nasty temper on the effects of alcohol. He admits to his bouts of rage and violence to the point that he seems to be a stranger to his former self:
...my general temperament and character—through the instrumentality of the Fiend Intemperance—had (I blush to confess it) experienced a radical alteration for the worse. I grew, day by day, more moody, more irritable, more regardless of the feelings of others. I suffered myself to use intemperate language to my wife. At length, I even offered her personal violence.
Later on in the story, he says that he then mutilated his cat Pluto because the cat had bitten him lightly on the hand. However, this time he culled out the cat's eye in a cold, calculating way that seemed more perverse in its indifference than his initial outbursts of anger. For this reason, it seems as if the narrator is undergoing some kind of character transformation, often associated with a bi-polar personality type (or perhaps a neurosis degrading into a full-fledged psychosis). We see where this finally leads him - to the murder of his wife and futile attempt at concealing the crime. This emotionally fragile person ends up as a bona fide sociopath who feels no remorse except for that of getting caught.