Answer:
A good claim would be something along the lines of the main theme presented in the story is the deep void that is insanity.
Step-by-step explanation:
You could back it up by pointing out evidence that proves he is not completely sane (such as him hearing the beating heart through the floorboards or him watching the old man whilst he sleeps). To back it up, follow through with why it proves him to be insane and connect it modern day standards and defintions of insanity/madness