In this story, the narrator keeps claiming that he is not insane. I do not believe him even a little bit.
First of all, his behavior seems insane. It does not seem sane to me to kill someone for no reason and then freak out because you think you can hear their heart beating and condemning you.
The second thing that makes me think he's insane is the way he talks. His sentences are so short and they sound so frantic. It just really seems like he's crazy from the way he talks.
I loved the old man. He had never wronged me. He had never given me insult. For his gold I had no desire. I think it was his eye! yes, it was this!
In Poe's short story "The Tell Tale Heart" the narrator begins by tying to convince the readers that he is not crazy. He reiterates this message many times throughout his story. The presentation of the lines indicate a sense of distress.
"TRUE! - nervous - very, very dreadfully nervous I had been and am;"
The man tries to convince the reader that he loved the old man, but in the same breath he shares that the man's eye was at fault for his need to kill the man.
"He had the eye of a vulture - a pale blue eye, with a film over it. Whenever it fell upon me, my blood ran cold."
The man goes on to tell about his insomnia and the sounds he hears in the night.
"He was still sitting up in the bed listening; - just as I have done, night after night, hearkening to the death watches in the wall."
The man has murdered the old man, but now he keeps hearing his heart beating. he even thinks that it is so loud that the neighbors may have heard it.
"And now a new anxiety seized me - the sound would be heard by a neighbor!"
The sound of the heart intensifies but the policemen in the house do not seem to hear it. They continue talking and laughing having heard nothing, but the man is sure they are mocking him.
As the sounds increase the words repeat and also increase in size. This indicates to the writer that the man is experiencing more stress.
"I felt that I must scream or die! and now - again! - hark! louder! louder! louder! louder!"
The narrator is definitely mad!