What is the message of "The Black Cat"?

This excellent short story by Edgar Allen Poe seems very similar
to other examples of his shorter fiction, especially perhaps "The Tell-Tale Heart." The main
similarity is its exploration of evil and the divide between madness and sanity depicted in the
unreliable narrator who tells us this tale. This story the narrator begins with a preface in
which he deliberately states that he is not mad, whilst the narrative that follows suggests
otherwise:

For the
most wild, yet most homely narrative which I am about to pen, I neither expect nor solicit
belief. Mad indeed would I be to expect it, in a case where my very senses reject their own
evidence. Yet, mad am I not--and very surely do I not
dream.

In spite of his protestations,
he then calmly relates how, thanks to drink, he tortured a favourite pet and then, after killing
it, was haunted by a similar, if not identical pet, who goaded him into "a rage more than
demoniacal" which resulted in him killing his wife. At every stage we question how reliable this
narrator is, and whether, in fact, he is sane or insane. Thus it is that one of the central
themes or messages of this tale is the way in which Poe deliberately blurs the distinction
between sanity/insanity and explores the mind and psychology of a killer.