Whatever "thousand injuries" could be caused by Fortunato to Montresor, they most probably do not correspond to the cruel nature of the revenge act. Otherwise Fortunato, if he had truly done something horrible to Montresor, would definitely know that, and would not trust Montresor too much. Besides, judging by the amount of human bones in the vaults, Fortunato was not the first to insult Montresor and to be killed by him. The way Montresor "renders justice" is truly horrifying as well - he is calm