(Vox) "On some level, the appeal of a mystery story is getting to solve the case alongside the protagonist. The best mysteries lay out all the evidence before your very eyes, but only make sense once you put that evidence together in a certain way, one that the author has deviously hidden from you. The worst mysteries intentionally withhold key evidence so you could never possibly guess the solution. In other words, they cheat. (For more on this, I recommend this excellent, though very long, video takedown of the BBC series Sherlock.)
True crime, in contrast, usually presupposes that you know the ending right from the start. For the most part, true crime stories conclude with someone behind bars, even if there’s some question as to whether that person actually committed the crime (as in, say, Making a Murderer or Serial). Rather than give viewers a mystery to help solve, they let viewers get to know the very worst of human nature from a safe distance. It’d be impossible to make a TV series where the writers tried to keep the identity of Jeffrey Dahmer a secret — his story is simply too well known. But a true crime show about the serial killer might aim to delve into his psychology, or the gruesomeness of his crimes, or the story of how he was caught, or all of the above.
A combination of these two formats is what makes Discovery’s Manhunt: Unabomber so intriguing. On the one hand, viewers know who the Unabomber is — though the show introduces one Ted Kaczynski very early in its run, just to be sure. On the other hand, much of the series is played like a mystery, where the characters have the evidence in front of them but don’t yet understand how it all fits together." Learn more at Vox