The narrator's description of Mrs. Mallard shows someone who brushes off the notions of love and even the best of marriages for the glorious idea of pure freedom. Meanwhile, the people around her think she's crying her eyes out over her dead husband. Really, though, she's relieved to be free. No one understands her. At the end of the story, the doctors agree that she must have passed away from a sudden shock of extreme happiness from finding out that her husband lived after all: from "joy that kills" (23).
Only readers and the narrator know that Mrs. Mallard had already experienced a potentially "monstrous joy" (12) earlier, alone with the knowledge of a lost spouse and the free life stretching out ahead of her. Which "joy" is more "monstrous"? And what is it that really killed Mrs. Mallard? Was she happy? Surprised? Horribly disappointed? Well, the narrator leaves that question up to you.