Montresor is the protagonist and narrator of "The Cask of Amontillado" by Edgar Allan Poe, and he claims from the beginning that the reason he had to seek revenge is that he was insulted. We do not know until the story unfolds exactly what that means, but he expresses his motive clearly in the first line of the story:
The thousand injuries of Fortunato I had borne as I best could, but when he ventured upon insult, I vowed revenge.
Here is the problem: Montresor is not a reliable narrator. What he says may have some truth in it, but we cannot trust his judgment. If the insult were serious enough to warrant being murdered, a "sane" person would not have been able to act as if nothing were wrong until he had the opportunity to arrange a murder.
It must be understood that neither by word nor deed had I given Fortunato cause to doubt my good will. I continued as was my wont, to smile in his face, and he did not perceive that my smile NOW was at the thought of his immolation.
If the insult were serious enough, the one who gave the insult (Fortunato) would not have been on such friendly terms with the man he insulted so strongly (Montresor).
While it is clear that Montresor is perfectly capable of planning and executing a well-staged murder, he is not rational about his reason for doing so. He accepted "a thousand injuries" before this one insult, and yet the one insult was enough to prompt a murder. That is not rational thinking.