I like your title! What you need to do now is to re-read the short story and consider what particular line you are going to take. I think it is pretty clear if you read the short story that the narrator's behaviour is largely the result of his alcoholism. That in fact could be your thesis statement. Now you just need to identify quotes that support this idea, such as this one:
One night, returning home, much intoxicated, from one of my haunts about town, I fancied that the cat avoided my presence. I seized him; when, in his fright at my violence, he inflicted a slight wound upon my hand with his teeth. The fury of a demon instantly possessed me. I knew myself no longer. My original soul seemed, at once, to take its flight from my body and a more than fiendish malevolence, gin-nurtured, thrilled every fibre of my frame.
You can hopefully see how this quote supports your thesis statement. We can see the barbaric behaviour of the narrator as being directly linked with his alcoholism. What he does to Pluto, in cutting out his eye, is something that he does because of being drunk. The way he describes himself makes it clear that it is almost somebody else who is committing this crime. Note that he says "I knew myself no longer," and "the fury of a demon... possessed me." This quote clearly supports the thesis expressed above. Now all you need to do is find more quotes to support your ideas. Good luck!