Why do you think the author leaves the motivations of his characters unclear in "The Cask of Amontillado"?
I cannot agree that Poe leaves the motivations of his characters unclear. Montresor wants to kill Fortunato. His reasons for wanting to do so may be questionable but not his desire to commit a murder without being caught.
Fortunato's motivation needs examination. Montresor tells him he just bought a "pipe" (126 gallons) of Amontillado at a bargain price. Montresor knows his man. It is not the wine but the bargain that interests Fortunato and motivates him to insist on sampling it that very night--in spite of his bad cold and in spite of the fact that he is unsuitably dressed for venturing into the cold, damp catacombs.
Montresor embroiders his falsehood in such a way that Fortunato feels he must act immediately. Neither man intends to drink 126 gallons of Amontillado. Both are obviously dealers in expensive things, such as paintings, antiques, and what Montresor calls "gemmary." Wine improves with age if stored in an oak cask. Fortunato knows he could buy the entire shipment and sell it off in bottles over a long period of time, making a big profit. But Montresor repeats that he has his doubts about the wine's authenticity and needs an expert opinion-- suggesting that, since he is looking for verification post facto, he intends to buy more of the wine if he can be assured it is genuine.
Once Fortunato learns--or thinks he learns--that a shipment of Amontillado has recently arrived in Venice, he doesn't really need to sample from Montresor's cask. He can easily find the source, which would be a recently arrived Spanish ship in the harbor, and deal directly with the captain. He could sample the wine on board.
But Montresor has foreseen that risk. He says he is on his way to Luchesi that very night, since he was unable to find Fortunato. If Luchesi finds out about a cargo of Amontillado available at a bargain price, he could go directly to the ship himself. Fortunato would find himself competing with another man and both bidding up the price. So Fortunato has to go with Montresor to keep Montresor from going to Luchesi.
Otherwise, Fortunato would most likely have pleaded a prior engagement and gone directly to find the Spanish ship. He is a rich man and, unlike Montresor, could afford to buy up the whole shipload of wine. Montresor is equally subtle and crafty. He knows, from prior experience, what he could expect from his friendly enemy if the Amontillado actually existed. Fortunato would taste it, frown, shake his head, and say it was only ordinary sherry.
Then, assuming it was genuine, he would go post haste to find the Spanish ship. Montresor would learn eventually that Fortunato had bought up the entire cargo; but Fortunato would treat that as "an excellent jest." It would be another injury to add to the "thousand injuries" Montresor had already suffered from his more affluent business rival.
Fortunato is not wearing a jester's costume because he wants to be taken for a fool. He regards himself as a jester, a man who enjoys playing cruel jokes. All the time he is following Montresor through the catacombs he is planning to judge the wine as ordinary sherry. There are two reasons why he doesn't ask questions about it while they are walking to Montresor's palazzo and winding their way through his catacombs. Poe has provided Fortunato with a bad cold and cough, making it hard to talk. But Fortunato does not want to show too great an interest in the source or price of the nonexistent wine. He is just doing a friend a favor.
The reasons for Montresor's desperate desire to kill Fortunato in "The Cask of Amontillado" still puzzles readers today, and Poe may well have hoped for this type of reader response. It is more than a gothic/horror story; it is also a mystery in which the reader must play detective. Poe leaves few clues for Montresor's reasons for murder aside from him having to bear "the thousand injuries of Fortunato." It is clear that Montresor's hate for Fortunato is strong, and the reader must wonder what connection the two men had with one another and how Fortunato really felt about his murderer. The uncertainties about Montresor's "true nature" also suggests the possibility of insanity, adding to his already unreliable narration. There are too many unanswered questions and missing details for the reader to ever completely embrace Montresor, who succeeds in committing the ghastly yet bloodless perfect crime. The unknown element adds to the suspense, and it raises other questions ("To whom was Montresor telling his story?", "Why was Montresor confessing now?") that will always be open for debate.
It must be understood that neither by word nor deed had I given Fortunato cause to doubt my good will. I continued as was my wont, to smile in his face, and he did not perceive that my smile now was at the thought of his immolation.