A new PBS documentary tries to excuse a murderous and totalitarian cult. When his captors uncinched the noose around his neck and shoved him into a wooden chair, Alex Rackley might have assumed his ordeal was over. He had already endured a flurry of kicks and punches, the repeated crack of a wooden truncheon, ritual humiliation, and a mock lynching. But it wasn’t over. It was about to get much, much worse.Rackley, a slight, 19-year-old black kid from Florida, was tough (he had a black belt in karate), but hardly in a position to resist his psychopathic interrogators. During a...