Why does the narrator spend so much time in the "tell tale heart" attempting to justify the murder?

Hi,

The narrator spends so much time in "The Tell-Tale Heart" attempting to justify the murder of the blind man because he is insane; he is a madman. The narrator of the short story tells the story in a calm way to try to prove his sanity. He tells us at the end of the story that he is not mad.

The narrator of the short story is, in fact, mad. He has a nervous disease.

Faith xoxo