What makes for a good crime documentary?

There have been many great true-crime dox, beginning probably w/ The Thin Blue line and reaching fruition in some of the miniseries such as OJ Made in America and esp Making of a Murderer. Roughly, there are three elements that move a crime doc from good to great: a great and intriguing crime, a narrative that sustains mystery and ambiguity, and use of much original footage. The HBO doc from 2016, Mommy Dead and Dearest by all means has a weird and intriguing crime: a young woman w/ severe illnesses and disabilities is accused of conspiring w/ a young man, possibly her boyfriend, to murder her mother in cold blood. The twist is - and I'm not really giving anything away as you learn the entire essence of the case in the first 15 or 20 minutes of the film - the young woman has succumbed to her mother's perverse and abusive behavior and over the course of her whole lifetime has faked these illnesses and disabilities. Her mother had her, among other things, pretend to be a cancer patient (even shaved her hair) and pretend to be unable to walk w/out a wheelchair. In fact she had her daughter undergo a lifetime of painful and invasive medical procedures - and as a result the mother was lavished with praise and affection as a mom carrying a heavy burden and the were even provided w/ gifts from charities and donors (a Habitat for Humanity house, trips to Disney, etc.). The young woman - actually in her 20s and quite articulate, though she was presented as a myopic teenager with mental retardation - enlisted help of an online boyfriend and had the mother killed, a crime she denied at first and then admitted. OK all very strange, and the film raises a ton of questions about who's responsible for this travesty: where were the social services? where was the school system? what about the doctors and others who administered 24 years of unneeded medical interventions? The problems with this film, though, lies in the other two elements. First of all, there's no sense of mystery; we know the basic facts within a few minutes and then learn that the young woman - Gypsy Rose Blancharde - had been forced to pose as someone with crippling illnesses. My thought is the film would have worked better had we "met" Gypsy earlier in her life and then build toward the crime, the arrest, and the unraveling. Second,  unfortunately for the filmmakers, there isn't a lot of original footage - recordings of the police interviews and and of some courtroom material, for the most part - so the doc consists 80 percent of talking heads. It's almost as if the quality of a documentary is inverse to the % of talking-head interviews. The team did a good job with the material in hand, but it does fall a little short of greatness.