The point of view is provided by one of the main characters, Montresor, who is seeking revenge on the other main character, Fortunato, so the story has a first-person narrator and point of view. Unfortunately, Montresor is so consumed by the desire for vengeance, he is an unreliable first-person narrator, which means we must be careful about how we interpret almost everything that Montresor does.
The major theme in the story is revenge and how the desire for revenge can lead a person to do something horrific to another person. Because we never know exactly what Fortunato has done to Montresor to create such anger, we have to question whether or not Montresor is even remotely justified in his actions.
Your question gets to the heart of Montresor's revenge: It is not good enough just to murder Fortunato. The murder--the revenge--must be accomplished in such a way that Montresor has no chance of being caught. That's why Montresor has carefully planned to commit the murder in the catacombs under Montresor's palazzo where no one can possibly witness Fortunato's end. Montresor has succeeded in murdering Fortunato, and there is no likely way that Montresor can ever be caught--he has murdered with impunity.