How might the final line in the story, "In pace requiescat," meaning "Rest in peace," apply to Montresor as well as Fortunato?

This line applies to both characters having closure with one another. When Montesor had the intention of killing Fortunato, his goal of having to getting rid of the "bad feelings" is also another common way of having "closure" with one another.

"Rest in peace" is a common expression used to wish a deceased soul eternal rest. This expression would apply to Fortunato, who has been entombed alive. "Rest in peace" also applies to Montresor, as he had been angry about Fortunato’s perceived insults. After getting revenge against Fortunato, Montresor’s troubled mind and hurt feelings will be able to rest peacefully.