The narrator in this story is suffering alcoholism. He becomes intoxicated and becomes hideously violent toward all in his presence. It started as he began mistreating and ill-using them. This held true for all his pets except for Pluto. This is strange considering his early description of himself  in youth as being "noted for the docility and humanity of my disposition." Now he tells us that he is abusing his pets. Alcohol is to blame for the condition that he was in the night of the attack on Pluto. He returned home very drunk that evening and "fancied that the cat avoided [his] presence". He proceeded to snatch Pluto up and cut out his eye. All for his drunken feeling of being avoided. This seems a turning point for the narrator as well.