How does society come across as an antagonist in “The Story of An Hour” and “The Yellow Wallpaper”?

In the two stories you ask about, "The Story of an Hour" and "The Yellow Wallpaper," society is the source of stereotypes, mores, and gender roles.  The protagonists in both of these stories live in patriarchal societies.  As women, they are supposed to fit feminine stereotypes, act in certain ways, value certain things, and play specific roles.  When a woman does not fit the stereotypes, believe in the same mores, and perform the "correct" roles, she is outcast, possibly in a literal sense (as in "The Yellow Wallpaper"), but certainly at least in a figurative sense (as in "The Story of an Hour"). 

These thinking, feeling, expressive women are doomed, doomed by the societies in which they live.  In this sense, society is an antagonist in both stories.