Seeking to shine some light into the dark world of Internet trolls, a journalist with Finland’s national broadcaster asked members of her audience to share their experience of encounters with Russia’s “troll army,” a raucous and often venomous force of online agitators. The response was overwhelming, though not in the direction that the journalist, Jessikka Aro, had hoped. As she expected, she received some feedback from people who had clashed with aggressively pro-Russian voices online. But she was taken aback, and shaken, by a vicious retaliatory campaign of harassment and insults against her and her work by those same pro-Russian...