In film after film, from “Nobody Knows” to “Shoplifters,” Japanese master Kore-eda Hirokazu has proven himself to be among the medium’s most humanistic directors, inclined to see the best in people, especially children. So how to reconcile the way “Monster” makes us feel?

Returning to his native Japan after helming two relatively disappointing features abroad (“Broker” and “The Truth”), the 2018 Palme d’Or winner opens his latest Cannes competition entry with a building on fire — a “hostess bar” where lonely men seek female company — and fifth-grade Minato (Kurokawa Soya) watching the inferno from a nearby balcony. Kore-eda will return to this scene three times over the course of the film, folding the narrative back upon itself from a different angle each time, before finally revealing who set the blaze.

The title misleads for a time, inviting us to speculate about the darkness that surrounds young Minato. Could a child have been the culprit?...