Final answer:
Montressor seeks revenge on Fortunato because he could no longer tolerate the injuries and insults received, aiming to punish him in a manner that is both clear to Fortunato and safe for himself.
Step-by-step explanation:
Montressor seeks revenge on Fortunato because he has borne the 'thousand injuries' Fortunato inflicted upon him and when Fortunato ventured upon insult, Montressor vowed revenge. Montressor's desire for retribution is rooted in his feeling that a wrong is unredressed 'when retribution overtakes its redresser' and equally unredressed 'when the avenger fails to make himself felt as such to him who has done the wrong.' Therefore, Montressor's motivation is to not only punish Fortunato but to do so in a way that is clear and recognizable to Fortunato without putting himself at any risk of retribution.