The most important statement about the human condition in "The Cask of Amontillado" is on how shallow and empty revenge truly is. While Montresor commits his perfect crime, by the end of the story he shows dissatisfaction with his plan:
My heart grew sick; it was the dampness of the catacombs that made it so. I hastened to make an end of my labour. I forced the last stone into its position; I plastered it up.
(Poe, "The Cask of Amontillado," eNotes eText)
Montresor no longer takes joy in Fortunato's fate, although he tries to blame his "sick heart" on humidity. In truth, he is feeling the first pangs of remorse, but refusing to recognize it. Even though he "gets away with it" for fifty years, the joy of committing his crime wears off and he feels compelled to confess it to the reader. It is possible that he is burdened by the murder, but more likely that he simply cannot take as much pleasure in committing the crime as he did in planning it. His revenge takes a few hours and is over; all benefits from it afterwards require too much mental justification to be worth it.