The word you are seeking from Edgar Allan Poe's short story, "The Cask of Amontillado," is impunity. Impunity is defined as being exempt, or having no fear, of punishment for an action that may be taken. Thus, one of Montresor's conditions for carrying out his murderous plot against Fortunato was that he must "punish with impunity." Revenge, according to Montresor, was not complete unless the avenger avoided being punished himself--by arrest, injury, or death. This was a primary reason that Montresor took such careful precautions in order to have no possible witnesses around when he fulfilled his vengeful plot.
Yes, "impunity" is the word. The protagonist has a terrific problem in luring Fortunato off the streets without being recognized himself. He must also guard against being suspected after Fortunato's disappearance. That is why he has consistently behaved in the most friendly manner toward his enemy and why he continues to call him "my friend," "my poor friend," etc.