Answer:
I would invent an organism that can "invade" the human body and break down faulty cells (similar to the role of lysosomes in cells); however, in doing so, it would release CO2 into the bloodstream and causing the victim to asphyxiate. I would call it "sumptus bona venimus" (meaning "good things come at a cost"). The monster trait would be that the organism is microscopic and can give false hope to the victim only to later cause the victim to die.