Montresor is an especially effective enemy to Fortunato, because Fortunato completely trusts Montresor.
A person could be an effective villain by being supremely powerful or by having some amazing awesome plan. But being an effective villain is made harder, if anybody that you are trying to be a villain toward is always suspicious of you. I imagine that Darth Vader has a really hard time being a sneaky villain. He looks suspicious, sounds suspicious, and has a history of being evil.
Montresor has none of Darth Vader's problems. Fortunato assumes that they are still "buddy-buddy." Fortunato has no previous encounters with Montresor in which Montresor made Fortunato's life miserable. Basically, Fortunato has no reason to suspect Montresor of anything except honesty and good will. And Montresor knows it.
Montresor also knows exactly what to say and offer to Fortunato to get him to follow Montresor anywhere. Montresor name drops "Luchesi" to goad Fortunato. Montresor also knows that the allure of an Amontillado is too great of a temptation to be ignored.
Any episode of CSI would fit the bill for this kind of "bad guy," because in that show the bad guy is always the best friend that you would least suspect. I have a film example. It's the movie "Predestination" with Ethan Hawke. It's based on the short story "All You Zombies." It works as an example because the main protagonist and antagonist know each other completely . . . because they are the same person. There are lots of weird paradoxes in the movie, since time travel plays a big part.