When Brently Mallard enters the house alive and well in the final scene, his appearance is utterly ordinary. He is “a little travel-stained, composedly carrying his grip-sack and umbrella.” His mundane appearance contrasts greatly with Louise’s “feverish triumph” and her walking down the stairs like a “goddess of Victory.”
When the doctors determine that Louise “died of heart disease — of joy that kills,” the reader immediately recognizes the irony. It seems clear that her shock was not joy over her husband’s survival, but rather distress over losing her cherished, newfound freedom. Louise did briefly experience joy — the joy of imagining herself in control of her own life. And it was the removal of that intense joy that led to her death.