Marriage is presented by Chopin as an institution that stifles women's freedom by subjecting them to the control of their husbands. There's no suggestion that Mr. Mallard has been a bad husband; it's simply that the very nature of marriage inevitably entails his being in control of his wife. That's the general expectation of society, and as a decent, respectable member of the middle-classes, Mr. Mallard unthinkingly goes along with the prevailing conventions.
Furthermore, Mrs. Mallard's heart condition merely adds to the hopelessness of her situation. In fact, one could see her bad heart as a symbol of the relatively weak position which women are forced to adopt inside marriage. Even if Mrs. Mallard wanted to go out into the world and do her own thing, she wouldn't be able to, not just because of her weak heart but also because of society's notions of a woman's proper place. This is reflected in Mrs. Mallard's final heartbreak on seeing her husband walk through the door, very much alive and well. It seems that there's just no escape from the institution of marriage and the complete level of control it exerts over women.