Who or What is responsible for killing Mrs Mallard? In 'The story of an hour'

While “responsibility” for Mrs. Mallard’s death is a stretch of the events of Kate Chopin’s "The Story of an Hour," it is definite that her death resulted from a complication of her heart condition. She died of a “joy that kills.”
The irony of that statement, a “joy that kills,” is that just before Mr. Mallard walked in the front door, unscathed, Mrs. Mallard was the most joyful that she had ever been. However, when she witnessed him, she released a piercing scream that relates to the reader that Mrs. Mallard was horrified that her newfound freedom had been so easily ripped right from under her feet.
Chopin writes "The Story of an Hour" via the unique perspective of the late 1800’s in America, wherein it was expected that women would be married before they reached a certain age. Our deceased Mrs. Mallard was described as “young,” meaning that although her marriage was a relatively new one, it was still one that had taken a very large toll on her health.
An important passage from "The Story of an Hour" demonstrates the immediate positive influence Mr. Mallard’s departure from her life has on Mrs. Mallard’s health:

"There was something coming to her and she was waiting for it, fearfully. What was it? She did not know; it was too subtle and elusive to name. But she felt it, creeping out of the sky, reaching toward her through the sounds, the scents, the color that filled the air… Her pulses beat fast, and the coursing blood warmed and relaxed every inch of her body.”

This relaxation is what would have likely saved Mrs. Mallard’s life. Stress is known to be a common factor in many heart ailments. Furthermore, as the American Heart Association writes in their article ‘Stress and Heart Health,’ “When stress is constant, your body remains in high gear off and on for days or weeks at a time.”
All in all, it is contextual that the stressful experience of losing her highly desired freedom was the catalyst for Mrs. Mallard’s heart failure and death.
https://my.hrw.com/support/hos/hostpdf/host_text_219.pdf