The narrator mentions having a "spirit of perverseness" twice after he cuts the eye out of his black cat. He explains it as a feeling of contrariness: of desiring to do what he knows is wrong just because it is wrong. He rationalizes it as one of the "primitive impulse of the human heart" and a "primary" sentiment. In other words, he universalizes it, not as his problem alone but as a problem for the whole human race, an inborn or innate desire to do evil. As he puts it,

Who has not, a hundred times, found himself committing a vile or a silly action, for no other reason than because he knows he should not?

It is this spirit of perverseness, he says, that causes him to hang the cat he has already mutilated. He describes this as something he feels sad about doing even as he is doing it. He says he is crying as he does it and does it because he knows the cat loves him and has done nothing to deserve this fate.

As we can see, this spirit of perverseness has a terrible and antisocial effect on the narrator's personality, causing him to act out in cruel and destructive ways. In earlier times, he might have been seen as in the grip of original sin or a demonic force. In modern times, Freudianism might interpret his spirit of perversity as the return of the repressed, in which his id (unsocialized self) breaks loose to enact its aggressive desires on creatures and humans that can't fight back.