The narrator changes from a quiet, compassionate child to a madman, prone to uncontrollable fits of rage and--in his own words--perversion in adulthood.
He tells us that even from infancy, he was "noted for the docility and humanity of [his] disposition," being so compassionate that his peers mocked him. He particularly loved pets, and his parents got him a variety of them growing up, and this is an affection that followed him into adulthood. Apparently, the pets felt the same about him because after he married, he and his wife collected pets, among them a black cat named Pluto.
Over time, he changes. He blames this on alcohol. He becomes "more moody, more irritable, more regardless of the feelings of others." He turns to insulting his wife then to beating her. Eventually, he turns his "fiendish malevolence" upon Pluto, cutting one of its eyes from the socket one night. The cat, when it heals, runs from him (of course!), and at this point, the narrator admits that in a fit of uttter perverseness, he hangs the cat by the neck from a tree.
Eventually, the cat is replaced by an eerily similar one which gets on his nerves, too. When he tries to kill it with an ax and his wife stops him, so he turns on her and kills her with it. Once he disposes of the body, he is finally able to sleep in peace (after murdering his wife!), simply because the cat is no longer following him around.
He has thus progressed from a gentle child to a violent psychopath.