Before she was confronted by a TV news crew, and before she was found dead in a hotel room near her home, 63-year-old Brit Brenda Leyland allegedly devoted much of her ample free time to tweeting abuse at the parents of a toddler who went missing seven years ago.

Those parents, Kate and Gerry McCann, were long ago absolved of any guilt in their daughter Madeleine’s case, which dominated European headlines for months. But that didn’t convince people like Leyland, who sent more than 4,000 tweets to the McCanns between December 2010 and this year, sometimes writing more than 50 tweets a day.

“I fear, that we are in this 4 long haul,” she tweeted on Sept. 17, as if four years weren’t a long haul enough. “Up to all of us to a) Bang home the facts b) Make #McCanns live in shame for years.”

That type of sustained vitriol might seem sort of deranged — certainly, there’s plenty to suggest that the more severe trolls among us suffer from something more serious than righteous vengeance. But as crazy as Leyland’s behavior might seem, it’s not terribly uncommon: There are hundreds of Barbara Leylands. Thousands of them, even. There’s an entire population of people who exist, it would seem, solely to foment outrage and speculation around tabloid cases that have long since concluded or gone cold.

More than 600 people belong, for instance, to the Facebook page “Casey Anthony Boycott Information,” a group dedicated to sharing hysterical, minute updates on the personal lives of anyone even tangentially involved in the case, including Anthony’s parents, attorney and “village idiot jurors.” (Current topic of fascination: the potential foreclosure of George and Cindy Anthony’s house.) A page on Lisa Irwin — the 10-month-old who disappeared from her home in Missouri almost exactly three years ago — still updates almost daily, even though there are no developments in the case. (Page administrators are convinced Lisa’s mom, who was never even charged, is the culprit.)

Meanwhile, defenders of Amanda Knox have for seven years waged a pitched battle for control over the Wikipedia page on her case — even establishing a pro-Knox wiki of their own. And those same defenders claim to be battling an army of anti-Knox trolls, who have manipulated Wikipedia and lobbied crowdfunding sites to take down Knox’s fundraisers. The hashtag #AmandaKnox still sees thousands of tweets a week.

“The Wiki War illustrates how social media is being influenced,” the administrator on a pro-Knox Facebook page wrote just three weeks ago, “so we want to extend our thanks to the efforts of the supporters who are behind this website.”

To outsiders, perhaps the Knox crusaders and Leylands of the Web look like bored, hopeless trolls; to the trolls themselves, though, they’re fighting for a just cause — they’re vigilantes in the wilderness, fighting murderers and kidnappers when the justice system has failed. In many ways, the line between the people we call “trolls” and the people we cheer for bringing crimes to the public’s attention — think Anonymous, or various armchair detectives — is a very, very thin one. In both cases, activists feel they’re acting to correct a crucial imbalance of power. And in both cases, it’s terribly easy to overstep balance … and turn into a lynch mob.

This, ultimately, is what makes Leyland’s case so tragic: She was ultimately undone by the same forces she sought to harness. Leyland, for years an armchair Twitter investigator, soon found herself investigated by supporters of the McCanns, who thought her tweets constituted harassment. They, in their role as power-balancing Internet vigilantes, compiled a dossier on her, the original Internet power-balancer. That dossier made its way to a Sky News reporter, who confronted Leyland outside her home.

Days later, Leyland was dead in a hotel room; authorities ruled her death “not suspicious.”

Perhaps this should be a lesson to all the other cold-case trolls and vigilantes out there: When anyone can take justice into their hands, everyone becomes vulnerable. But that doesn’t mean anybody will stand down. In Leyland’s fateful words to Sky News, “I’m entitled to do that.”