What is the "joy that kills" in "The Story of an Hour" by Chopin?

At the end of this story, Louise Mallard dies of "heart disease -- the joy that kills," according to her doctors.  However, after reading the story, readers can ascertain Chopin's irony: Louise did not die of joy, she died of the terrible shock of seeing her husband alive when she'd believed him to be dead. 

When she'd learned of her husband's death in a train accident, she repeated the word, "'free, free, free,'" again and...

At the end of this story, Louise Mallard dies of "heart disease -- the joy that kills," according to her doctors.  However, after reading the story, readers can ascertain Chopin's irony: Louise did not die of joy, she died of the terrible shock of seeing her husband alive when she'd believed him to be dead. 

When she'd learned of her husband's death in a train accident, she repeated the word, "'free, free, free,'" again and again,"'Free!  Body and soul free!'"  She did not rejoice in the death of a man who "never looked save with love upon her"; rather, she rejoices that his death would supply to her "a long procession of years to come that would belong to her absolutely."  She anticipates the independence she would possess as a widow, one who would not have to bend her will to meet anyone else's.  The narrator describes her as feeling a "monstrous joy," likely because it comes at the cost of her loving husband's life but provides her with a freedom that makes her pray "that life might be long" when it "was only yesterday that she had thought with a shudder that life might be long." 

Therefore, when her husband, Brently Mallard, opens the front door and steps through, just as Louise descends the stars, "carr[ying] herself unwittingly like a goddess of Victory," her husband's friend tries to shield Brently from Louise's sight.  When she collapses, doctors believe it to be connected to her apparent "heart trouble"; however, though "Her pulses beat fast" when she first learned the news, instead of weakening her, "the coursing blood warmed and relaxed every inch of her body."  In fact, the narrator's claim that the lines on her face "bespoke repression and even a certain strength" seems to indicate that she had been weakened by "repression," a social ailment, rather than any physical one.  Thus, it is not a joy upon finding Brently to be alive that kills her, it is the revocation of the "monstrous joy" she felt when she thought him to be dead.