The Bird with the Crystal Plumage (1970)

an american journalist investigates on his behalf after casually witnessing the wounding of a young woman and learning that the mysterious stabber is perhaps responsible for other crimes. after being threatened several times, the journalist discovers the origin of the threatening phone calls: the house in which the young woman he had seen wounded lives. the husband, surprised with a knife in his hand, while he seems to want to kill his wife, confesses to being the murderer before dying, falling into the void. but the truth is different: the murdering paranoid is instead the wife, who, for love, had always been protected by her husband.