After hearing the news of her husband's death, the first reaction that we get from Mrs. Mallard is that she "wept at once, with sudden, wild abandonment," which seems to indicate that she was horrified at the news, and miserable as a result of it. In this reaction, Louise is playing the protagonist; she is reacting normally, we feel for her grief, and get wrapped up in the misery that she must be feeling. Our hearts go out to her, and we automatically rally to her side, championing her cause. That is typically the reaction that the protagonist, or "good guy" in the story gets. We like them, or at least are on their side.
However, as soon as she enters her room, she experiences a bit of a change of feeling. The story states,
"There was something coming to her and she was waiting for it, fearfully...She was beginning to recognize this thing that was approaching to possess her, and she was striving to beat it back with her will."
Here, she undergoes a serious change. There is a feeling overcoming her, almost like a normal person who changes into a superhero or a villain. It is a feeling that she is dismayed by, but can't fight back. That feeling is freedom, elation, and joy that her husband is dead. Once she realizes what that feeling is, and how powerful it is, she surrenders to it, rejoicing in the fact that
"There would be no one to live for during those coming years; she would live for herself. There would be no powerful will bending hers in that blind persistence with which men and women believe they have a right to impose a private will upon a fellow-creature."
So, she goes from the grieving widow that we feel bad for, to a heartless creature who is rejoicing in her husband's death. Whether we empathize with that feeling or not, it is quite a change, and all of a sudden she is a woman feeling "monstrous" emotions, and for some, it might feel like she has turned into the antagonist, or the "bad guy" of the story. I believe that was what your question was referring to-Mrs. Mallard is both the protagonist AND antagonist in the story, because she stands on both sides of the major event of the story, her husband's death. I hope that those thoughts help a bit; good luck!