The first thing you need to determine is what events in the story show that the narrator is insane. One event that might show his insanity is the obsessive nightly visits he makes to the old man’s room with a lantern that shines a ray of light onto the old man’s eye. The fact that he hallucinates he hears a beating heart under the floorboards is another reason to claim insanity as a defense.
I would start your essay with your claim and then answer the question, why?
For example, here is a possible thesis--
The narrator in “The Tell Tale Heart” is innocent due to insanity because of his obsessive actions and hallucinations.
Then back it up with your examples and proof. Be sure to conclude your essay with a statement that restates your argument’s claim.