Marriage is the true antagonist in the story. Mrs. Mallard has no real issue with Mr. Mallard. He was not an abusive or unkind husband, and she does have feelings of sadness that he is dead (as she believes). She recalls that she even loved him sometimes.
What she objects to, and is thrilled to be liberated from, is the assumption that she must constantly, in many subtle ways, conform her life to her husband's. She has had to constantly think of his needs. He has not been cruel to her, but the set-up of the society they live in has nevertheless stifled her.
As she begins to realize what has happened, the joyous word "free" comes to her. She thinks:
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.
It is not Mr. Mallard as an individual who she objects to but the institution of marriage. One is tempted to think it is patriarchy which is the antagonist, and that too is implied, but what Mrs. Mallard is truly thinking of is the stifling confines of marriage as it was practiced in her time--she speaks not only of men, but of the "blind persistence with which men and women believe they have a right to impose a private will."
Mrs. Mallard simply wants to be free to be herself. Her conflict is that her society does not condone that path.