I grew, day by day, more moody, more irritable, more regardless of the feelings of others. (6)
These are the beginning stages of the man's hideous psychological transformation. He connects it to alcohol (though perhaps there is more going on). When alcohol and/or drugs play a central role in a story, transformation is probably featured as well. The nature of the transformation depends on the story. Here the transformation is altogether negative.
I knew myself no longer. (7)
Most people experience this feeling at some point or another, especially when in the midst of changing. Here the narrator is stressing the degree of transformation, hoping we remember that he was a nice guy in high school.
I took from my waistcoat pocket a pen-knife, opened it, grasped the poor beast by the throat, and deliberately cut one of its eyes from the socket! (7)
This is a transformative moment for both Pluto and the narrator. The narrator crosses a previously uncrossed line of violence. His act is irrevocable. Pluto's physical transformation will be a constant reminder of crossing the line. Eye problems and eye violence are often featured in stories of transformation. They signal transformed vision in the characters, and hope to signal transformation in the reader.
The destruction was complete. My entire worldly wealth was swallowed up, and I resigned myself thenceforward to despair. (9)
While the man focuses on the transformation of his material circumstances, readers might focus on the transformation of the pets that were left to burn up in the fire. The transformation from alive to dead is often a prominent fixture of horror stories.
The reader will remember that this mark, although large, had been originally very indefinite; but, by slow degrees -- degrees nearly imperceptible[…] it had, at length, resumed a rigorous distinctness of outline. (21)
The mark, as you know, is an image of the gallows, both recalling the murder of Pluto, and foreshadowing the man's death sentence. It's interesting because it suggests a gradual transformation, and introduces the possibility of the supernatural. Along with the eye situation, want the reader to believe he was convinced that the cat has supernatural qualities. What do you think?