Let us remember that the story is told from the third person limited point of view, zooming in on the thoughts and feelings of Madame Loisel. Because of this, her opinion dominates. What is interesting about this short story, therefore, is that there is actually no character or person who Madame Loisel is in conflict with. Actually, the antagonist would be the expectations and desires of Madame Loisel herself that she is constantly wrestling with. Consider the following quote:
She grieved incessantly, feeling that she had been born for all the little niceties and luxuries of living. She grieved over the shabbiness of her apartment, the dinginess of the walls, the worn-out appearance of the chairs, the ugliness of the draperies. All these things, which another woman of her class would not even have noticed, gnawed at her and made her furious... She would dream of silent chambers, draped with Oriental tapestries and lighted by tall bronze floor lamps, and of two handsome butlers in knee breeches, who, drowsy from the heavy warmth cast by the central stove, dozed in large overstuffed armchairs.
Although actually she is economically stable as a middle-class French woman, her desire for more and her feeling of entitlement for more is what drives this darkly ironic short story by Maupassant. By the end, at least part of us can't help feeling that she has certainly got what she deserved for her pretensions and her inability to be grateful and accept what she has.
Mathilda vs Self