Montresor describes the mounds of bones and stench of human remains so elegantly, it almost sounds beautiful. The following passage is a good example:
We passed through a range of low arches, descended, passed on, and descending again, arrived at a deep crypt, in which the foulness of the air caused our flambeaux [torches – pronounced “flam-bow”] rather to glow than flame.
Read it out loud. Doesn’t it sound pretty? See how it moves? See how “flambeaux” rhymes with glow? Perhaps the beauty makes it scarier.
The creepily humorous tone also adds to our engagement in the story. In addition to being entertaining, Montresor’s sinister (and usually somewhat lame) jokes (like the one about how he gets his servants out of the house in paragraph 24) make us believe, for a moment, that everything is going be OK. If we can still laugh, it must not be so bad. When things get rough for Fortunato, we feel a little guilty for having laughed before.