Alex Hyde-White (Sean) is the son of Wilfrid Hyde-White who appeared in the film 'Ten Little Indians', an adaptation of the Agatha Christie novel that inspired this film
Early in the movie, when Miranda is in the book store, she opens the book to a chapter titled "Locard's Exchange Principle," named for Edmond Locard, a French criminologist commonly regarded as the "father" of forensic science in the early 1900s. The principle says that "every contact leaves a trace." Thus, whenever two objects come into contact, there is always an exchange of material between them; e.g., if a person is hit on the head with a tree branch, some trace of the branch will be left on the head and some trace of the head will be found on the branch.
Detective Church says that the killer will spend the rest of his life in a cell. As this is set in Great Britain in 1934, the killer could very likely receive the death penalty.
The film takes place ten years before And Then There Were None was published.