[…] through the instrumentality of the Fiend Intemperance [I] had (I blush to confess it) experienced a radical alteration for the worse. (6)
So much for plain language, though readers in Poe's day would have been familiar with "the Fiend." This expression is a common way of representing addiction. The desire to drink takes on a life of it's own. It has "instrumentality."
One night, returning home, much intoxicated, from one of my haunts about town, I fancied that the cat avoided my presence. (7)
What's interesting to us here is the word "fancied," which is similar to "imagined." The narrator is being tricky here. We can't tell if Pluto actually avoided him, or if the narrator imagined it. This moment seems significant when we consider the extreme affection of the second cat toward the narrator. Why doesn't that cat avoid him?
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. (7)
In plain language, the narrator's "soul" leaves his body (he doesn't say where it goes). "Malevolence" is the desire to do evil. In the passage above, the desire to drink is shown as a fiend. Here, malevolence is even worse than a fiend. If what left the man's body was his "original soul." This malevolence might be a new one.
[…] the soul remained untouched. I again plunged into excess, and soon drowned in wine all memory of the deed. (8)
Well, we know the reason the man's soul wasn't touched by guilt over the cutting out Pluto's eye. Because his soul is worse than a fiend, and drunk on gin. And now wine. Things don't look good.
One morning, in cool blood, I slipped a noose about its neck and hung it to the limb of a tree -- hung it with the tears streaming from my eyes, and with the bitterest remorse at my heart. (9)
We bring this up here because it seems to be a moment when the man isn't drunk. He doesn't blame the murder of Pluto on the alcohol. Do you think this makes it more or less scary?
One night as I sat, half-stupefied […] my attention was suddenly drawn to some black object, reposing upon the head of one of the immense hogsheads of gin, or of rum […]. (14)
Enter cat number two. Or reenter Pluto. You could make an argument for either interpretation of the second cat. "Stupefied" means drunk. Hogsheads are big barrels for holding alcohol. This is the last we hear of drinking in the story.