Montresor told his servants that he would not be home, so they should not leave, because he knew they would leave.
Montresor knew that his house would be empty because he made sure that his servants would leave by telling them that he would not be returning home. His servants don’t respect him. He is certainly not up to Fortunato’s station in life. He knows how to get rid of his servants.
I had told them that I should not return until the morning, and had given them explicit orders not to stir from the house.
If you tell people that you won’t be home, so they should not leave—chances are they will leave. After all, you won’t be there to catch them! So they all go off and make merry. It’s carnival season! Everything is hectic, and most people are just out having a good time. It is the perfect opportunity to kill someone without raising suspicions.
Montresor chooses carnival time because he knows his servants will leave the house the minute his back is turned and they know he won't be coming back to catch them gone. It is all part of his brilliant plan to have an alibi in case anyone wonders where he was. It takes awhile to lure someone into catacombs and bury him in the wall, after all!
Montresor is a rather pathetic figure because he is poor and lives all alone in a huge palazzo. He has no wife or children, although he might have had a family who died. Since he is a poor man, as he tells Fortunato when they are down in the catacombs, he probably has a hard time retaining any staff of servants at all.
"Come," I said, with decision, "we will go back; your health is precious. You are rich, respected, admired, beloved; you are happy, as once I was. You are a man to be missed. For me it is no matter."
When he orders his servants to remain in the palazzo while he is gone overnight, he is not exactly using reverse psychology. Not all servants would behave as Montresor's servants have done. His servants don't respect him because he doesn't pay them enough money. And furthermore, he can't afford to hire the best class of servants. There must be times when he can't pay his staff anything. They stay on because they have a roof over their heads and get something to eat. (They probably also get plenty of wine to drink!) The servants do not "abscond" because Montresor tells them to stay at home; they do so because he tells them he will be gone all night. They don't care if he finds out that they all left to join in the carnival. He can't fire them because he can't get anybody to replace them. And such an enormous old dwelling needs some servants to keep it from ruin. And if he should fire them, they wouldn't have lost anything.
How does Montresor obtain any income at all? Why does he choose to live in an enormous palazzo when he is all alone in the world?