From my infancy I was noted for the docility and humanity of my disposition. My tenderness of heart was even so conspicuous as to make me the jest of my companions.

This quotation is the narrator’s description of himself before alcohol, a sensitive and peaceful man who loves animals. These character traits demonstrate the narrator’s sense of self—whom he believes himself to be before he falls into alcoholism. The stark contrast between this self-image and the man he becomes over the course of the story suggests a split personality, with the self and the alter ego contained in one body. However, it’s worth noting that the narrator describes himself as “tender of heart,” or sensitive. The narrator’s sensitivity, when combined with the selfishness his drinking causes, makes the narrator more violent.

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.

The narrator here describes the transformation that occurs when he believes Pluto is avoiding him. He treats the violence that overcomes him as a kind of possession, as if he’s becoming another person. He speaks of his original soul leaving, implying that a literal new personality has taken its place. In the narrator’s understanding of himself, who he becomes when he drinks is separate from him, a demon or spirit that possesses him. However, this violent person is still him.