Montresor specifies that he must murder with "impunity" in order for his revenge to be totally successful. He knows there is sure to be a big investigation into Fortunato's mysterious disappearance. It could actually go on for years. If the police authorities gave up on it, Fortunato's family might still pursue the investigation. It would long be the talk of the town. Montresor wanted to be above suspicion. That explains why he calls Fortunato "my friend" and why he refers to him many times throughout the story as "my friend," "my good friend," and "my poor friend." He has conditioned himself to think of Fortunato as his very good friend even though he plans to murder him. No doubt he customarily refers to Fortunato as "my friend Fortunato" at every opportunity. And no doubt Montresor continued to ask about his good friend for years after his disappearance, longer than anybody else in Venice. He had concealed his victim behind a stone wall and plastered it over to make it look like part of the solid rock wall of the catacombs. Nevertheless, he would have preferred not to be included in any house-to-house search. He assumed that no one would ever suspect him of having murdered Fortunato because they were such very good "friends." He was probably right in this assumption. During the fifty years that have passed since he chained his victim to the rock wall, it is unlikely that anyone has passed by that spot, with the possible exception of Montresor himself. Montresor has not only committed his crime without being caught in the act, but he has managed to avoid any hint of suspicion that he could have had anything to do with the strange disappearance.
Many questions have been asked about the "thousand injuries of Fortunato." Readers wonder why Poe didn't at least give a few examples. Poe had to be vague about the “thousand injuries” because he had to protect Montresor from suspicion. Perhaps the injuries would have to be of such a nature that they would be known only to Montresor. Fortunato himself might not have considered them injuries at all but as "jests." Montresor is obviously a hypersensitive man who might feel slight injuries more poignantly than others. The fact that Montresor has endured a thousand injuries should not be surprising if the injuries were so slight and so subtle that nobody else was aware of them and everybody thought the two men were the best of friends. In other words, if the injuries were petty, there would have to be a lot of them to motivate Montresor to retaliate.
Poe actually gives samples of the kinds of injuries Montresor has suffered. Here is one:
He raised it to his lips with a leer. He paused and nodded to me familiarly, while his bells jingled.
“I drink,” he said, “to the buried that repose around us.”
Fortunato knows that all these bones are not those of Montresor's ancestors. Montresor's French name shows he is a relative newcomer to Italy. He doesn't consider himself Italian. He probably doesn't own the palazzo but is renting it from the owner, who is also the owner of most of the bones. Fortunato is being disingenuous. The "leer" shows he is being intentionally cruel in reminding Montresor of his inferior social status.
Here is another example of Fortunato's injuries:
“I forget your arms.”
“A huge human foot d'or, in a field azure; the foot crushes a serpent rampant whose fangs are imbedded in the heel.”
“And the motto?”
“Nemo me impune lacessit.”
Fortunato does not believe that Montresor has a coat of arms. Here again he is being disingenuous. In fact, Montresor's family does not have a coat of arms or a motto, and Montresor is only making them up to amuse himself. Both the imaginary coat of arms and the motto are too appropriate to the situation. Fortunato may or may not believe they are genuine, but he is too drunk to give them much thought at this time.
Why has Montresor continued to associate with Fortunato all this time if he keeps receiving injuries and contumely? Perhaps the two men have some business relations which are important to Montresor because Fortunato is rich and well connected. The third paragraph of the story suggests that both these men deal in one-of-a-kind luxury items such as oil paintings, jewelry (gemmery), and antiques. Perhaps even in real estate. They are not shopkeepers, by any means, but might be described as "gentlemen brokers."
He had a weak point—this Fortunato—although in other regards he was a man to be respected and even feared. He prided himself on his connoisseur-ship in wine. Few Italians have the true virtuoso spirit. For the most part their enthusiasm is adopted to suit the time and opportunity, to practise imposture upon the British and Austrian millionaires. In painting and gemmary, Fortunato, like his countrymen, was a quack, but in the matter of old wines he was sincere. In this respect I did not differ from him materially;—I was skilful in the Italian vintages myself, and bought largely whenever I could.