Mrs. De Ropp dies when she is attacked by Conradin's polecat-ferret, Sredni Vashtar. The story of how she dies is faithfully described by the author and is presented as a case of poetic justice.
Accordingly, when Mrs. De Ropp demands to know if Conradin is keeping guinea pigs in the tool-shed, he doesn't apprise her of the truth. So, she marches down to the shed to see for herself, after she retrieves the key from Conradin's bedroom. It is clear that Mrs. De Ropp aims to clear out any offending articles she finds in the shed.
Meanwhile, Conradin watches her from the dining-room window. He prays that Sredni Vashtar will rise up against his oppressor. However, even as he prays, Conradin despairs that his nemesis will ever be defeated. He imagines that, after she finds Shredni Vashtar, her triumph over him will be complete. The gardener will be tasked with carrying away Conradin's prized pet, and he, Conradin, will once more be subject to Mrs. De Ropp's "pestering and domineering and superior wisdom" until he succumbs to an early death.
However, his hopes rise when he sees not Mrs. De Ropp emerge from the shed but the ferret:
Hope had crept by inches into his heart, and now a look of triumph began to blaze in his eyes that had only known the wistful patience of defeat. Under his breath, with a furtive exultation, he began once again the paean of victory and devastation. And presently his eyes were rewarded: out through that doorway came a long, low, yellow-and-brown beast, with eyes a-blink at the waning daylight, and dark wet stains around the fur of jaws and throat.
The "wet stains" allude to the fact that Mrs. De Ropp has not survived her encounter with Shredni Vashtar. This fact is further confirmed by the maid's screams and someone's pitiful lamentation about not being able to break the news to the "poor child."