In "The Story of An Hour," who or what is the antagonist and what is the story's central conflict?

In The Story of an Hour, the antagonist would be Mr. Brentley Mallard, Mrs. Mallard's husband.  The central conflict revolves around Mrs. Mallard being informed that her husband has been killed in a train accident.

After learning of this news, Louise Mallard goes upstairs into her room and sits in a chair facing a window and begins to imagine the life she will have as a widow.  Up until that moment, she never imagined that she would ever have any individual freedom in her life, since, in this period in history, women were considered the property of their husbands and could not go out without them.

"Chopin deals with the issues of female self-discovery and identity in "The Story of an Hour." After Mrs. Mallard learns of her husband's death, she is initially overcome with grief. But quickly she begins to feel a previously unknown sense of freedom and relief."

Mrs. Mallard has lived a life that has largely been controlled by Mr. Mallard's wishes and choices.  She has loved her husband, she tells the reader, most of the time, but she has not loved the confining feeling that marriage has given her. She longs to make her own decisions, choices, to explore her own interests, just the thought of having the ability to make such choices makes her feel joyful.

"Louise is ecstatic when she realizes that ''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." 

Her joy, however, is short-lived, when at the end of the story she gets a great shock, a surprise that causes her to die on the spot, her husband walks in the front door, very much alive.  All the free days she imagined she would have, sitting upstairs in her chair, thinking of herself as a widow, free from the demands of marriage, are gone in an instant.  In that same instant, Louise Mallard's weak heart gives out and she dies. 

She dies a happy death, having had the opportunity to explore, even if it is only in her mind, what life would be like freed from the constraints of marriage and the control of her husband.  

"Chopin seems to be making a comment on nineteenth-century marriages, which granted one person—the man—right to own and dominate another—the woman. This theme, unpopular in an era when women were not even allowed to vote, is examined in many of Chopin's other works, most notably The Awakening."