"Innonence ruined." That seems to be the common blurb on this popular Henry James novella. Daisy Miller, though, was never innocent -- but delightfully manipulative -- and her ruination in death (as clankingly melodramatic as Miles' death in The Turn Of The Screw) is a needless moral lube job slathered over the cogs of the plot.
What of her death? Well, James, as elsewhere, wants to eat his cake while having it permanently enshrined in a culinary museum. "Innocence" must be brought down, but instead of acting with anger (rather than bemusement) towards his voyeuristic, sycophantic, death- (not eros-) "stiff" (as Daisy Miller perfectly pegs him) creation of Winterbourne, James shirks the courage of his emotional conclusion by having the young lady's suitor Giovanelli cause her fatal contraction of malaria by the "foolish" meeting at the Colosseum. The new, moneyed aristocracy must be chastised, but it remains for the lower-rung social hangers-on to be the fall guys.
A moral is only effective if it's at the service of a tragic, or at least sympathetic, dilemma. Daisy Miller is well-written, and has a great deal to say about the clashing of the barbarians -- the old aristocracy with their moneyed counterparts -- but James flinches when both, or either, have to be fatally excoriated. The amoral Italian commoner has the plot-dagger placed in his set-up hand.