Why does Montresor tell his servants to stay in the house even though he knows he will not return until the next morning?
In Poe's "The Cask of Amontillado," there are a couple of insights that can be gleaned from Montresor's admission that his servants do not obey him as shown in the following passage:
"I had told them that I should not return until the morning and had given them explicit orders not to stir from the house. These orders were sufficient, I well knew, to ensure their immediate disappearance, one and all, as soon as my back was turned."
First, Montresor reveals in this passage that he is a man whom people do not respect. If his own servants can get away with disrespecting his orders, and Montresor knows this, then he must be viewed by many people as a weak person. Because Montresor allows others to disrespect him, then one might infer that Fortunato, Montresor's enemy, and victim, is one of many who makes fun of him publicly. Fortunato must be the most vocal of those who treat Montresor poorly; as a result, he becomes the scapegoat for Montresor's offended feelings.
Second, from the information above, we know that the murder of Fortunato is premeditated. For example, Montresor does not suddenly become offended because of a mindless or drunken insult said during a party and then decide to kill Fortunato. Montresor has been thinking about this murder for quite some time; and by ensuring that his servants will not be home, Monstresor ensures there will not be any eye-witnesses to his crime, which is certainly needed to pull off a murderous plan.
Montressor tells the reader that his servants are not very respectful toward him and that if he tells them to do something, they will do the opposite. Therefore, Montressor specifically tells his servants that they are not to leave and then tells them that he will be out for the entire night. This basically ensures that the servants will leave the house during the night.
Montresor knew fully well and understood the character and nature of his servants: they were a disobedient lot and would do exactly the opposite of what he orders them to do. So he orders them to stay at home during the carnival, knowing fully well that as soon as he leaves the house, the servants will also abandon their duties and go take part in the carnival. Consequently, no one will be in the house when he lures Fortunato into his house to commit the diabolical deed of murdering him: "I had told them that I should not return until the morning and had given them explicit orders not to stir from the house. These orders were sufficient, I well knew, to insure their immediate disappearance, one and all, as soon as my back was turned."