I forced the last stone into its position; I plastered it up.
This death suggests the mental torment that Fortunato inflcited upon Montresor. It is indeed so gruesome and tortuous to bury a man alive that it can only be saved for someone guilty of something truly devious. Then he ends his deeds with "In Pace requiescat" which translates to "may he rest in peace". It is ckear he feels that he has fulfilled his vengeance and is free of his torment by knowing that Fortunato had suffered. There is nothing more completed in Gothic than a horrible death in a creepy tomb.