Why it is significant that this cat will not leave the narrator of "The Black Cat" alone?

After the
narrator kills the first cat, following gouging out one of its eyes, a second black cat appears,
also with a missing eye. It is huge like the first cat but differs in that has a white mark on
its chest.

The significance of the second black cat not leaving the narrator
alone is that the narrator comes to hate and fear this new cat terribly. The narrator is very
upset when he decides the white mark on it is in the shape of a gallows. Further, this new cat
constantly torments him by reminding him of his guilt over the cat he abused and killed. This
guilt keeps him from abusing this new cat, but his sense of dread grows and grows.

Because the shape of the gallows on the cat's breast seems like a punishment from God
and because the cat is always around, reminding him of his wrongs, the narrator is unable to
rest. Finally, he gets to the point where he can't stand it anymore and tries to kill the cat
with an axe. He misses, hits his wife's hand, and then sinks the axe into her brain.

From the narrator's point of view, if the cat had not constantly been with him,
haunting him, he would not have been tormented to the point of swinging the axe that killed his
wife.

We have to imagine the narrator is more than a little delusional, but
the constant presence of the cat acts as a catalyst to his criminal
behavior.