Even those who have not read the Harry Potter books, or seen the movies, know that the tone grows relentlessly darker as the series unfolds.
I was reminded of this tonight as I started Book Four, "Harry Potter and the Goblet of Fire," which begins with the discovery of three murder victims. It's like opening an adult Whodonit novel, compared to the usual high-satire of the Dursley's that earlier books begin with. J.K. Rowling could easily have eclipsed Agatha Christie, one suspects, if she had chosen to write in this genre.
In describing the murder scene involving three members of the Riddle family, Rowling also convincingly sketches a scenario showing why the presumption of innocence is needed in criminal cases. When Frank Bryce, the Riddles' gardener, is arrested, public opinion in the village of Little Hangleton soon swings against him. But when new evidence emerges, the unproven (and very possibly erroneous) nature of the assumption of Bryce's guilt is unmasked.
If the new evidence had not come to light, however, it's hard to see how Frank Bryce could have found an impartial jury anywhere in the vicinity of Little Hangleton.