If Montresor were only in his twenties at the time of his revenge against Fortunato, he would be in his seventies after "fifty years." So, there is no reason to not boast about his perfect crime since he is probably near death, anyway. (Few people lived past their seventies in Poe's time.) To whom he speaks is unknown, of course, but it may be someone of Montresor's ilk who would appreciate such a crime; that is, someone who also wishes to avenge himself.
Certainly, Montresor is quite proud of his undetected plan designed in order to avenge himself against Fortunato. Perhaps, not only does he wish to boast of his masterful and successful plan, but he may wish to instruct, as well, as he outlines his philosophy of why a wrong is "unredressed." Certainly, he seems to delight in proferring his strategic steps toward revenge and proving that he has successfully mastered these himself:
- The plan must involve no risk.
- The avenger must punish with impunity.
- The retribution must not overtake the redresser.
- The avenger must make himself known to the man who has wronged him