In a recent interview with The Verge, director Wingard spoke about how the movie ends, however, he hasn’t given too much away at the same time. “We wanted the ending to feel open-ended, so the reason we didn’t show anything is
As Wingard’s film is about a teenage boy who discovers a magical book, which allows him to control who lives and dies. It’s possible to assume what the context behind Wingard’s comments is. For example, Light (the teenager who finds the book played by Nat Wolff) sets out to rid the world of criminals, however, the usage of the book's power has a nasty side effect. There is a demon known as Ryuk (played by William Dafoe) who has made a number of rules for the usage of the book. Basically, once you start killing, you can’t stop, as Ryuk makes sure Light continues killing. Ryuk finds death amusing and if Light disobeys a rule, there will be terrible consequences. It is possible that whatever happens at the end of the film, is one of those consequences.
Since Illustrator Ohba released his story, the manga has inspired an animated TV series, several games, animated films and as well as live-action adaptations.