A group of journalists and researchers wade into ugly corners of the Internet to expose racists, creeps, and hypocrites. Have they gone too far?

We’ve come up with the menacing term “troll” for someone who spreads hate and does other horrible things anonymously on the Internet. Internet trolls are unsettling not just because of the things they say but for the mystery they represent: what kind of person could be so vile? One afternoon this fall, the Swedish journalist Robert Aschberg sat on a patio outside a drab apartment building in a suburb of Stockholm, face to face with an Internet troll, trying to answer this question. The troll turned out to be a quiet, skinny man in his 30s, wearing a hoodie and a dirty baseball cap—a sorry foil to Aschberg’s smart suit jacket, gleaming bald head, and TV-trained baritone. Aschberg’s research team had linked the man to a months-long campaign of harassment against a teenage girl born with a shrunken hand. After meeting her online, the troll tormented her obsessively, leaving insulting comments about her hand on her Instagram page, barraging her with Facebook messages, even sending her taunts through the mail.