Why Did Henry James Kill Daisy Miller?

“Here comes my sister! She’s an American girl.”  

Randolph Miller in Henry James, Daisy Miller, 1878.

The American girl is different. Daisy Miller horrified European America and much of Europe with her cheerful boldness, so Henry James killed her with a villainous miasma. Why?

There are two stories in Daisy Miller. First, the comedy of manners: an heiress goes to Europe and shocks American-European high society with her free-and-easy, potentially lewd American ways. Second, the public-health story: an heiress goes to Europe and dies of malaria. James mushes these two not-very-gripping stories together:  if heiress A is the same person as heiress B, the feeling of meaning emerges.