Alcohol and Descent into Cruelty
In Edgar Allan Poe's "The Black Cat," the narrator explicitly blames his behavior on alcohol. He describes his descent into cruelty and violence as directly linked to excessive drinking, which warped his judgment and led to actions he would never consider while sober.
Evidence from the Text
The narrator speaks of an "evil influence" that possessed him and an indifference to right and wrong. His drinking created a "fiery and demoniacal thirst" that consumed him.
This theme of vice and its destructive influence on sanity is common in Poe's work.