In "The Fall of the House of Usher," which of the narrator's experiences in the mansion might have led to his end?
I suppose you mean the deduction that Roderick Usher's sister Madeline had been buried alive.
Actually, both the narrator and Usher himself heard scraping noises but attributed them to outside noises or even the groanings of the old house rather than to his sister's struggling to get out of the tomb. No intention or act of crime has taken place but simply a terrible mistake. There is an element of denial here. Such a far-fetched hypothesis would be too horrible to be true. Too late they both realize it is indeed the case.
Subconsciously, did Usher actually wish his sister to be dead? They had lived many years together in each other's sole company in a lone, isolated place. Perhaps Poe was insinuating an incestuous relationship by which both characters had fallen under a curse.
(In real life, Poe married his cousin. Although this was an acceptable practice at the time, perhaps he felt somewhat guilty. But then again, he was adopted, so the "ties that bind" would have been familial but not genetic.)