He explicitly states that his descent into cruelty and violence is directly linked to his excessive drinking. He describes how his drinking led to a "fiery and demoniacal thirst" that consumed him, warping his judgment and leading him to actions he would never have considered sober.
Here are some quotes from the story that illustrate this:
* "My life had been a long and in some measure a happy one; but for the last year or two an evil influence, whose origin I cannot trace, has possessed me, and, in its baleful shadow, my soul has been overshadowed and darkened."
* "I was, beyond all doubt, thoroughly well aware of the fact that what I had done was wrong—wrong in the eyes of both God and man; but I had grown to feel an indifference to any thing."
* "And now, from the bottom of this heart, from which a legion of fiends might have been imagined, I feel a gushing of unutterable love, a love both for the animal itself and for the ravenous and unbridled thirst for vengeance which it seems to embody."
The narrator's blaming his alcoholism for his actions is a common theme in Poe's work, where characters often grapple with the destructive power of vice and its influence on their sanity.