In "The Black Cat" why did the narrator initially restrain himself from maltreating the cat while maltreating the other animals?

It is
strange that he begins maltreating any of the animals at all, because in the beginning of the
story he goest to great lengths to explain that he was "especially fond of animals...With
these I spent most of my time, and never was so happy as when feeding and caressing them." 
However, later, he says that he develops a fondness for alcohol, and it altered his behavior for
the worse.  He states that he starts behaving badly towards his wife, even using "personal
violence" against her.  Then, he starts abusing the animals.  Initially, Pluto, his cat, is
spared this abuse.  The narrator states, "For Pluto, however, I still retained sufficient
regard to restrain me from maltreating him, as I made no scruple of maltreating the rabbits, the
monkey, or even the dog, when by accident, or through affection, they came in my way."  So
what keeps the narrator from maltreating the cat-at first-was that he cared for the cat too
much; he still had "sufficient regard" for him.  In the end however, even that
affection was not enough to keep the cat from being the victim of the narrator's awful abuse.