Eeeeeh, there are some things I disagree with. I don't think Montresor is a sociopath, he's just a person who is determined to do what he sees as necessary. It's alluded to that the Montresors were a powerful, numerous, and influential family, but by the time of his birth, the story's protagonist is seeing the declining fortunes of his family. Fortunado, however, is a newcomer to the political and aristocratic scene, and he's garnering more attention and favor than the Montresors. Even the name belies this. So it would be well within the protagonist's view that Fortunado is contributing to his family's decline. He talks about the injuries Fortunado did to him, perhaps without Fortunado even noticing, but it was when Fortunado began insulting the Montresors that the protagonist vowed revenge. An up and coming, newly-rich aristocrat insulting an already well-established family? If nothing was done about it to show respect, that would just be another failure for the family. It's also alluded to that Fortunado, like his countrymen, is a quack. He portrays himself as very well versed in wine, and says Lucresi can't distinguish Amontillado from Sherry, but Amontillado IS Sherry. Likewise, in the catacombs, Montressor gives an already-drunk Fortunado a bottle of De Grave. A very expensive wine. And Fortunado just downs it in one gulp, further offending the refined Montressor and cementing in his mind that this man is nothing but a drunkard and just playing at being knowledgeable. Finally, when he's walling Fortunado up, I think Montressor does indeed feel the weight of what he's doing... but he dismisses it and blames it on something other than his conscious.
That doesn't have anything to do with being a sociopath or not, a sociopath also does what is necessary in their mind. How exactly was this necessary? in fact his motivations are completely petty and sociopathic, and are his behaviors. You usually wouldn't brick someone up alive because they are gaining notoriety and wealth out of petty jealousy and hatred of him being undeservingly arrogant.
