This is certainly a worthy question, and one that is, indeed, puzzling. Perhaps, the reader should consider that many of Poe's narrators are, first of all, unreliable. There is, for instance, the unstable narrator of "The Tell-Tale Heart," who admits his nervousness and exhibits bizarre behavior and reasoning, and there is the hallucinatory narrator of "The Pit and the Pendulum. Also, there is the distorted mind of the narrator of "The Black Cat" who relates his horrific acts as a "series of mere household events" and "nothing more than an ordinary succession of very natural causes and effects."
So, if Poe's narrator in "The Cask of Amontillado" is similarly out of touch with reality, he may certainly be exaggerating when he declares that Fortunato has dealt him "a thousand injuries." Hyperbole, after all, is not uncommon for those who react emotionally, rather than intellectually. Certainly, he has the propensity for the baroque style in his speech and in his catacombs that possess a labyrinth of rooms that wind and circle around. Poe himself called his style arabesque; that is, a style of writing that exhibits "exotic disorder" and unpredictablity.
A distant relative of Poe, modern scholar Harry Lee Poe, wrote that grotesque means "horror," which is gory and often disgusting, and arabesque means "terror," which forsakes the blood and gore for the sake of frightening the reader.
A consideration of Poe's style of arabesque, then, may explain why Montesor continues his association with Fortunato. For, with arabesque there is also the repetition of ideas and disorder. In the narrative, for instance, Montresor stops repeatedly, pointing to the niter and toying with Fortunato's pride by telling him they will turn back in consideration for his health, and he will consult Luchesi. In his words, Montresor demonstrates this repetition,
We will go back. Your health is precious. You are rich, respected, and admired, beloved. You are happy as I once was. You are a man to be missed. For me is no matter. We will go back. You will be ill and I cannot be responsible. Besides, there is Luchesi---
Then, after fettering and walling in Fortunato, Montresor's actions further exhibit the disorder of his mind as he is surprised and "thrust violently back" by the screams of Fortunato. Still, he hesitates and trembles only momentarily until he replies to the shouts:
"I re-echoed, I aided, I surpassed them in volume and in strength. I did this, and the clamorer grew still.
Finally, when Montresor cannot elicit more screams from his victim, his "heart grew sick." Thus, it seems that Poe's narrator takes a perverse delight in torture, whether it be of others or of himself. Just as he feels badly that the screaming has stopped, Montresor masochistically may well have enjoyed the "thousand injuries" of Fortunato enough to continue them so that he could all the more enjoy his revenge,
For over half a century, no mortal has disturbed them. In pace requiescat!