Montresor describes himself to Fortunato as a poor man. He is evidently just getting by financially. He probably does not own his palazzo but only rents the big place. He has to keep a few servants because of the sheer size of the building, but he probably can't afford to pay them very much besides their rooms and board. Hence he has servants who are disrespectful, disobedient and indifferent, and he can't fire them because he couldn't get any who were better. Other servants in other establishments would not necessarily behave the way Montresor's have done. He is not commenting on human nature in general when he says that they all went out to enjoy the carnival just because he had told them he wouldn't be home and had ordered them to remain inside the palazzo. He is giving the reader yet another clue to his own personal financial and social condition. He does not mention the fact that there is no family at home, but it is obvious that he has no wife or children and is all alone in the world. The fact that he is living in a huge mansion means nothing in particular. Nobody wanted these old buildings because they were dilapidated, because they were expensive to heat, and because they required a staff of servants. Henry James explains all this in his excellent short story about Venice, "The Aspern Papers." Many old aristocrats who still owned palazzos only lived in a few rooms without any servants at all.

The narrator of "The Cask of Amontillado by Edgar Allan Poe is Montresor, a man who wants nothing more than to exact his revenge on a man named Fortunato. The plan is elaborate and takes place during Carnival; however, it will only work if he can manage to get rid of his servants. Because he knows them well and has chosen his time wisely (during the Carnival season), he thinks he can make it happen.

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.

He says he ordered them not, under any circumstance, to leave his house, even though he was going to be gone all night and into the next morning. In a bit of irony, Montresor knows that this is just the thing his servants need to hear in order to ensure that they will promptly leave as soon as their employer leaves. 

This is an ironic statement, then, which is a contrast between what he said and what he expected to happen. And, of course, they did just as he anticipated they would, leaving him free to pursue his plan to exact revenge on Fortunato.