Simply put, Montresor feels that the injustice done to him was great but that somehow Fortunato is adding insult to injury. The first line of the story states it best. "The thousand injuries of Fortunato I had borne as I best could, but when he ventured upon insult, I vowed revenge." This is no different from the modern day person that has felt slighted for far to long and takes up vigilante action.
Montresor feels that the injustic done to him was bad enough but at Fortunado's insult he for a lack of better words went off the edge. He felt superior and that he was teaching Fortunado a lesson.