The defense attorney would surely build a case on an insanity plea. After all, sane people do not typically plot murder against a person who has insulted them. Montresor claims that he had endured thousands of "injuries" from Fortunato and thus plotted to seek his ultimate revenge. The defense would likely argue that a sane person who suffered such "injuries" would simply remove himself from the presence of such an offensive person.
The prosecuting attorney would likely examine the efforts that Montresor takes to commit the murder. The murder is plotted well and carried out quite effectively. Montresor lures Fortunato to the place of his death via flattery:
We will go back; your health is precious. You are rich, respected, admired, beloved; you are happy, as once I was. You are a man to be missed. For me it is no matter. We will go back; you will be ill, and I cannot be responsible.
This reverse psychology (insisting they go back, with the intent of making Fortunato follow him further), combined with the flattery of making the man feel "precious...respected...admired," shows a sense of keen reasoning ability, and the prosecution would point to this and claim that it could not support a defense of insanity. The story also concludes fifty years after the murder, so the prosecution would likely point to that amount of time with no further murders as a sign that Montresor was not insane at the time of Fortunato's murder; he has proven perfectly capable of navigating society for five decades without further incident. Therefore, the prosecution would point to a targeted plot against the life of Fortunato specifically, not the workings of an insane mind.
The jury's decision should rest where you can find the most evidence, as that is how decisions are reached in courts. Can you prove through your analysis that Montresor's acts were intentional beyond "any reasonable doubt"? Here is the legal definition of first-degree murder:
An unlawful killing that is both willful and premeditated, meaning that it was committed after planning or "lying in wait" for the victim.
Examine your final evidence in light of this definition and determine whether this is a likely verdict.