An adaptation of Truman Capote's famous reportage. Natural born killers. Fascinated by psychology and therefore entirely of its era: the logic of leaving no witnesses strikes me as sound, or putting it another way, consonant with humans not being too bothered about things beyond their immediate vicinity (cf poverty, climate change, general insanity, and so forth). Or compare it to the recent wars and mass murders with motivations even more confused. The black and white spaghetti chronology left me cold. I couldn't help but think of it as the view from NYC.
Roger Ebert called out the artiness in 1968. He had another (more reactionary) go in 2002.