Montresor dislikes Fortunato because he has always been insulted by him, so in a way its almost like he is tired of being bullied by Fortunato. A person that was wronged will always seek vengeance, Montresor narrates this in paragraph one, "A wrong is unredressed when retribution overtakes its redresser" (Poe 1118). So basically Montresor wants to right whatever wrong Fortunato did to him. Impunity means freedom from punishment, for Montresor's situation impunity is ridding Fortunato's rudeness from his life all together.
