Montresor certainly has no legal argument for killing Fortunato: When it comes to his reasoning, it is simply a matter of revenge. Montresor tells us about "the thousand injuries" that Fortunato has bestowed upon the name of Montresor, unspecified acts that he has
... borne as I best could, but when he ventured upon insult I vowed revenge.
Montresor talks of being "avenged," but he never gives the reader a clue about what could have brought on such hatred and animosity. Certainly no insult or injury could warrant murder, but Montresor believes otherwise. Perhaps Fortunato has had some part in causing Montresor's extreme unhappiness, and his jealousy of Fortunato is obvious.
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.
Montresor's premeditative plan includes a two-fold clause: In order to satisfactorily fulfill his desire for revenge, he must both kill without being caught, and the victim must be aware of why he is being killed.
I must not only punish but punish with impunity. A wrong is unredressed when retribution overtakes its redresser. It is equally unredressed when the avenger fails to make himself felt as such to him who has done him wrong.
When the deed is done and Montresor tells his story, there is neither remorse nor satisfaction. His actions and reasoning are cold--immoral and unrealistic--and we can only wonder what final "insult" triggered this supreme act of revenge that resulted in a perfect crime.