Both are deep dives into horrific crimes where even the simplest questions turn out to have no easy answer.
In many ways, the film and the podcast are quite different. (We're talking here about "Serial's" original season that focused on Adnan Syed.) In the case of Syed, who remains in prison for the murder of former girlfriend Hae Min Lee, many people argue over whether he actually killed her or was framed.
There's no question who committed the crime in "Slenderman." Two 12-year-old Wisconsin girls, Morgan Geyser and Anissa Weier, admit that Geyser, with Weier present, stabbed Payton "Bella" Leutner on May 31, 2014. Their 12-year-old victim managed to survive 19 stab wounds, and it sounds like a miracle that she did.
As with "Serial," "Slenderman" takes rumors and half-truths we may have gleaned from news reports and weaves a rich, disturbing story out of what is actually a very complex case.
As the documentary makes clear, the girls weren't mad at Leutner or jealous of her. They'd had a birthday sleepover and roller-skated together the day before. They liked her.
Instead, as the girls made clear in chilling police-interview footage, they believed they had to kill their friend to please an internet horror legend known as Slenderman.
The first season of "Serial" still gnaws at listeners because, for many, there's no satisfying answer to the question of who committed the murder or why. "Slenderman" may gnaw at viewers because, even though those two facts are known, there's still no real explanation.
Those who want to blame the girls' parents will find two loving and aware couples, who sang with their toddler daughters and took their sons to Boy Scouts.
Those who want to blame the internet will struggle with that, too. The girls' parents monitored their online use, talked to them about what they saw online and knew about the Slenderman myth. But never foresaw this. No one imagined that two girls who seemed to live in the real world had a deadly fantasy in their heads that ran unchecked.
And because the girls admitted to the crime, how should they be punished? As with Syed's "Serial" season, viewers are bound to take sides. Is the state of Wisconsin's decision to try the girls as adults fair? Should we burn all the iPads and take online access away from everyone under 18? Can the victim and her devastated family ever truly receive justice?
The documentary comes closest to an explanation for the stabbing when it delves not into Slenderman's role, but into the complex world of mental illness. Geyser's dad has a history of schizophrenia. After the crime, Geyser was diagnosed with early onset schizophrenia. Weier was diagnosed with delusional disorder schizotypy.
That may give a clue as to why an internet meme that's just a fleeting interesting to most people could be twisted into a motive for attempted murder. As with "Serial," "Slenderman" leaves an array of questions that viewers will mull over long after the documentary fades to black.
"Beware the Slenderman," released this week, is available on HBO, HBO Go and HBO Now.
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