The Cask of Amontillado shows that mental illness and madness in general can cause little things to drive someone over the top. This is expressed in the story when the narrator states that Fortunado had insulted him a "thousand times". It is probable that he is exaggerating a great deal, and that Fortunado may have unintentionally said something that angered the narrator. I think that the Cask of Amontillado simply assumes that mentally ill people overthink things that are said or done. This leads them to feeling as if they had been insulted and shamed by that person.

While in some cases this may be true, I think that the story exentuates this more than is realistic. Most people wouldn't kill someone over a simple insult.