The book’s third section, “The Defender,” is about the love affair between a 32-year-old Brooklyn-based landscape architect named Lorri Davis, “who spent her days thinking about the gradations of rich people’s backyards, figuring out the optimal placement for Oscar de la Renta’s pool,” and Damien Echols, a high school dropout and Wiccan, convicted of murdering three kids in Arkansas. Lorri was educated and well off, while Damien grew up without “running water, heating or air-conditioning.” She admired that as a teenager he had changed his name from Michael to Damien, in honor of “a 19th-century priest who treated lepers in Hawaii.” She learned of his plight in 1996, at a screening of a documentary at the Museum of Modern Art. Three years later, after a robust prison correspondence, they married in a jail wedding. (Early on, “they speculated about whether they’d known each other in a past life.”)

Instead of pathologizing the allure of violent criminals to lonely women, Monroe focuses on the hurdles Lorri faced in getting Damien, whom she and many others believed to be innocent, out of prison. Lorri was aided in large part by the support she received on internet message boards. Monroe cleverly juxtaposes this positive story about the internet’s role in freeing a possibly innocent man with the most mind-boggling account in the book: that of a teenage girl who is radicalized by murder fantasies online.

[ A popular true crime podcast, “Crime Junkie,” removed episodes after allegations of plagiarism. ]

“The Killer” tells the story of two lonely teenagers — one rich, one less so — who meet online and plan, poignantly, a Valentine’s Day massacre at a mall. In just seven weeks, “Lindsay and James would come to feel that their meeting was part of some great cosmic plan.” Both were active in a Columbine-shooter-focused Tumblr forum. (“I could be your Eric,” Lindsay writes to James, referring to one of the Columbine shooters.) “Girls who’d been toddlers at the time of the massacre in Colorado had an easier time metabolizing the tragedy as an aesthetic: trench coats, violent video games, industrial techno, semiautomatic weapons … cute, misunderstood boys,” Monroe explains, a conclusion she draws after reading the couple’s voluminous chat record. Lindsay flies to Canada happy to go along with any plan, as long as she can wear high heels during the attack. She gets arrested at the airport.

By the end of the book, I found myself almost admiring the emotional plasticity of women who consciously scramble the logic of the predator-prey relationship in order to escape their unsatisfying lives. “Relationship” seems to be the key word. Throwing your lot in with the living embodiment of lethal peril — and swapping out the label “predator” for “boyfriend” — seems a surefire way to change your life. And isn’t that what we all want? Our lives to change?