Going by Montresor's treatment of Fortunato, Montresor is a cold, calculating, vengeful person. Montresor claims that Fortunato has committed "a thousand injuries" against him, but he gives no real details about any of these injuries. We don't know if the injuries are mental or physical, or if they are in Montresor's mind. When the two meet and start discussing wine, Fortunato greets him as a friend. So, as far as Fortunato is concerned, there is no significant ill will between them. Montresor's grudge is therefore something Fortunato is unaware of or it is something insignificant enough for him to have forgotten it. Or the grudge is something that Montresor has embellished or created in his own mind, allowing himself to feed into it, and dwell on his thirst for revenge.
As Montresor lures Fortunato to his death, he does so without a hint of remorse. In fact, he tells the tale fifty years after it has occurred. He still has no remorse. He is a vengeful to the point of being completely immoral.
Montresor comes from wealth and this is something he values. He values money, his extravagant estate, and most of all, he values his reputation. From this, readers might deduce that perhaps Fortunato spoke ill of Montresor in public or behind his back. We can only speculate as to how (or if at all) Fortunato wronged Montresor. What we do know from the context of the story is that Montresor prides himself on his wealth and his reputation. We also know that Montresor can hold a grudge for over fifty years and that he can commit murder without a hint of remorse.