When the reader tries to find out why the main protagonist has turned into an animal abuser and later into a murderer, the first possible and at the first sight visible reason is an alcohol. The first really terrible act was encouraged by an intoxication. It is obvious, however, that this was not the root cause, we do not know what triggered the narrator's radical character deterioration. He claims this change happened during the last several years, when he grew more moody and irritable, that the disease - how he calls his addiction - grew upon him.
One of the possible reasons for his alcoholism might be the fact that he used to be mocked when he was young. Because of that he was fond of animals, which never challenged his friendship, he loved taking care of them and caressing them. Maybe he realised he have never had a real human friends and his animals only reminded him of it. Or he simply wanted to take his upset about the mocking-experience out on someone and the animals and his wife were at his elbow.
Another reliable theory could be some mental disorder. It is known that experiences from one's childhood can strongly influence the development of various psychosis in one's maturity. There could be one minor event, which is not even mentioned in the prisoner's confession, which was the starting-up impulse for the disorder.
One of the last but not least reasons for his change of character might be some mysterious influence of cats. Those are considered to be magical animals with an arrogant behaviour, and if we accede the possibility of The Black Cat being a mysterious tale of...