You might want to analyse the ending of this excellent story, that has lots in common with Poe's other classic, "The Tell-Tale Heart." Having killed his wife, the narrator conceals the body in an extremely cunning location that he thinks will escape the detection of the authorities. When policemen do come around to investigate the disappearance, he shows them the cellar where his wife's body is stowed away with complete calmnesss and confidence:
Secure, however, in the inscrutability of my place of concealment, I felt no embarrassment watever. The offficers bade me accompany them in their search. They left no nook or corner unexplored. At length, for the third or fourth time, they descended into the cellar. I quivered not in a muscle. My heart beat calmly as that of one who slumbers in innocence.
In spite of this confidence, the narrator's calmness quickly gives way to nervousness and panic when he knocks the wall behind which his wife's corpse lies and the sound of the cat is heard, revealing his crime. Note how he responds:
But may God shield and deliver me from the fangs of the Arch-Fiend! No sooner had the reverberation of my blows sunk into silence, than I was answered by a voice from the tomb!
Thus we can see when the narrator discovers that he had buried the black cat with his wife's corpse, his confidence and calmness give way to panic and a state of nervousness.