He cant possibly be insane! First, to decide if hes insane we need to know the true definition of insane. The term Insanity is used when someone repeats the same actions over and over again but expecting different results every time. Montresor killed Fortunado only once and he knew what the result would be.
Feigning insanity? Feigning insanity is the simulation of Mental illness in order to avoid or lessen the consequences of a confrontation or conviction for an alleged crime. One example of this is of Mafia boss Vincent Gigante who pretended to have dementia and was often spotted walking around his neighborhood in his pajamas muttering to himself. But through many surveillance cams and testimonies it was proven that he was perfectly sane and ruled over his Mafia family with an iron fist!
The beginning lines of the story make it seem like hes confessing to the murder of Fortunado to a preacher, which is a reason in itself that hes sane due to the possible fact that he feels guilty and the other potential fact that he understands that him murdering Fortunado is wrong.
A quote to back this up is my heart got sick- on account of the dampness of the catacombs. Implying that he feels bad maybe even guilty for killing Fortunado and torturing him. Overall there appears to be more of a case for Montresors insanity. He has many of the characteristics of your typical insane serial killer.
He feels justified in the killing of Fortunado, appears to get satisfaction out of retelling the story of his revenge at the beginning of the story, smiles at Fortunados death and pretends to be worried about fortunado to give him a false sense of security, and he watches and taunts him while he dies. All of these make him seem insane, but Montresor is afraid he will get caught and is very careful to cover up his tracks. This would suggest that he is actually perfectly sane, and considering the fact that we do not know what wrong Fortunado committed it is difficult to say if Fortunado deserved this fate.
The possibility of Montresors insanity is more upfront, so we can obviously see it. But we can find evidence that he is sane when we look at the fact that all of it was extravagantly thought out and he went to great lengths to hide it.
After reading this story by Poe I feel like the mood was definitely creepy and leery but that Montresor was wrongfully accused and that he is completely sane.