There is always truth in fiction, possibly more than in history books. And, fiction often appeals to the imagination and is, therefore, entertaining. Poe's Gothic tale is one that creates horror, and it is not horror from supernatural beings as in many gothic narratives. The real horror is that which Montresor feels in himself: the terror of the mind that has engaged in horrific deeds.
It is based on a true story, at least to a certain extent. There was a body found walled up in a crypt in Italy. Poe clearly thought revenge was the reason. I think the story definitely tells us a lot about people.
Obviously authors write to entertain, but it seems as if that is rarely their primary motivation. "The Cask of Amontillado" is a criticism of human nature and the affects of revenge on lives. We're not told what was responsible for the drive for revenge, but it did take over the narrator's life. Took over his life to such an extent that he was still obsessed with it years later.
It's both, of course. Entertainment is derived from our morbid fascination with the apparently deranged man's actions. it's kind of like watching a train wreck--we see it coming but we're powerless to stop it, and we really can hardly believe what we're seeing. It's a little sick, I know, but it's human nature to want to watch or hear about these kinds of horrific events. (And id you've ever ridden or driven by an accident, you know I'm right because nearly everyone feels to the need to slow down and gawk--even if it's a gruesome and deadly sight.)
Honestly it depends on who is doing the telling. If you are going from the Poe stance, then we don't know why he wrote this unless we ask him - which is a little hard since he has been dead a while. In general, a lot of schools teach Cask in the Poe units because it is one of his short stories. I teach it and tell it as an example of suspense in short stories and some imagery as well. I do make sure that I point out how Montressor and Fortunato could have possibly resolved their conflict in a different manner and what lead up to the resolution of the story. The story itself is entertaining, but you can't help but to learn something from it.
I agree with #2. It's the same reason Stephen King's stories are so popular...almost all of Poe's creations dive into the darker side of human nature. We all have tendencies, and through Poe and King, and others...we can entertain those thoughts without really putting them into action. And, as #2 also mentions, there is almost always a lesson to be learned if we look closely enough.
I think that it is both. It is a story that is highly entertaining; if it wasn't, it wouldn't be so popular, even today. The entire thing being about a revenge-obsessed, merciless and scheming man is highly fascinating, and then add the bumbling and drunken Fortunado in his ridiculous outfit, and it's a great combination for entertainment and suspense. As they weave through the catacombs, we already know what Montresor's final intent is, so we are kept in suspense, constantly wondering where it is all leading. Montresor's amusement and thorough enjoyment of the spectacle is also fascinating; all of this serves for a very entertaining tale.
However, note Montresor's fading happiness at the end. Throughout most of his evil scheme, he is filled with a felicity and glee at what is going on; he rests a bit to further relish Fortunado's cries, he mocks Fortunado's screams. But at the end, this elation stops. As Fortunado stops speaking or making noise, Montresor states that "My heart grew sick...I hastened to make an end of my labour." He is feeling sick at heart, and hurries to finish the job and get out of there. This last dampening paragraph of the story seems to indicate that even though exacting revenge might feel really good at the time, the effects are not worth it, and regret will set in. So, along with its entertainment value, there is a lesson to be learned from the story also.
It's so entertaining in large part because of the truths recognizable in it. Little winks such as "My own fancy grew warm with the Medoc" make the larger actions of the characters -- who under normal circumstances would be too eccentric to believe -- ring true. In this way Poe can tell an entertaining story by magnifying common impulses (which, of normal course, are inhibited before becoming anything more). This is one of the defining aspects of the art of fiction, and, I think, what makes this particular story so popular.