Montresor is vague about exactly how Fortunato has wronged him. So the reader can only guess as to whether Montresor was justified in his revenge. Considering the harsh nature of the murder, it seems unlikely that Montresor's crime can be justified at all, especially when Montresor claims that the last straw was a mere insult. Whatever injustice(s) Fortunato has allegedly done to Montresor, this is what led Montresor to exact his revenge on Fortunato. Montresor notes this in the first sentence of the story:
The thousand injuries of Fortunato I had borne as I best could; but when he ventured upon insult, I vowed revenge.
Given this first line, we can only assume and suppose what the insult could have been. And if it indeed was just an insult, we might suppose that Montresor is too sensitive and/or had some other unknown motive for killing Fortunato. Whatever the insult was, Montresor became furious but calculating. Still in the first paragraph, he remarks:
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 the wrong.