According to Montresor, there are two elements which must be present for a crime, specifically of revenge, to be perfect. 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".
Also asked, what does it mean that he seeks to punish with impunity?
The crime can never be attributed to Montresor. If a person commits an act of revenge, the vengeance must never be connected to him. Punish with impunity---this means to punish someone without being punished for the act. Montresor must never be caught.
What kind of person has Fortunato shown himself to be?
According to Montresor, Fortunato is a man who has caused him a "thousand injuries" and who has wronged him numerous times. He never details, though, what Fortunato has supposedly done to him. We do know that Fortunato is a powerful man who is respected and also feared.