In the tale, The Cask of Amontillado by Edgar Allen Poe, why does Montresor feel he has the right to take justice in his own hands?

In context, Montresor believes 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."