According to Montresor in "Cask of Amontillado", what makes a perfect crime?
It isn't Montresor's recipe for a perfect crime that he addresses, but it is what Montresor feels is the perfect revenge. He wants revenge against Fortunato for "the thousand injuries" Montresor feels Fortunato has done to him. In the first paragraph, Montresor states what must occur in order for revenge to be successful.
"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."
First, revenge isn't successful if the person seeking revenge allows it to control him. Montresor must not allow his desire for revenge to consume him. Sometimes people want revenge so much that they destroy themselves in the process. Montresor says this must not happen. This is why he says he must "punish with impunity." He will make Fortunato pay for his insults, but Montresor will be free from any harm to himself.
The second part of successful revenge is that Fortunato must know that Montresor is the person who is seeking revenge against him. The person seeking revenge must let the other person know who he is. Montresor would never allow someone else to act in his place; he must committ his crime of vengeance himself.
According to Montresor, there are two elements which must be present for a crime, specifically of revenge, to be perfect. First of all, it "must not only punish, but punish with impunity". The avenger, or "redresser", must be able to get away with the crime without being punished himself - "a wrong is unredressed when retribution overtakes it redresser". The victim of the crime of revenge must also be fully aware that he is being punished for the crime to be perfect; if the victim does not know what is happening to him and why, the punishment is ineffective. The wrong for which the revenge being extracted is "equally unredressed when the avenger fails to make himself felt as such to him who has done the wrong".