Answer:
He literally went insane dealing with the cat. He is known to have mental illness anyways, and he is often despondent and depressed. He knows if he continues to drink, that he will probably do something he should not do. He drinks anyways. Kills the cat (murders the cat !), another cat comes - as if he has been raised from the dead - and this cat has a mark on him to make Poe think that the cat is speaking to him from the grave.
Explanation:
I learned that people who have addictions can go mad.
