The whole story is about revenge and what approach Montresor takes upon it. In the story, it says, “The thousand injuries of Fortunato I had borne as I best could, but when he ventured upon insult I vowed revenge.” This is saying that Montresor has dealt with Fortunato's foolishness before but it pushed him over the line when he insulted him. Montresor was not going to let Fortunato do that to him, and that is why he swore revenge upon the fool.
Another quote from the story is “Nemo me impune lacessit” which means “No one attacks me with impunity”. This foreshadows that Montresor is very angry about being insulted and that he will act in revenge. Karma came after Fortunato eventually. He couldn't get away with insulting people and especially not people like Montresor who will take action and get revenge.
The point of view in “The Cask of Amontillado” is first person unreliable narrator. It shows throughout the story that this tale is being told many years after the scene occurred. That means that nobody knew that Montresor did that to Fortunato until the story was told. In the story, it says, “Against the new masonry I reerected the old rampart of bones. For the half of a century, no mortal has disturbed them.” The bones of Fortunato were never disturbed and nobody ever found him after Montresor closed him