His incentive, it would turn out, was romantic love. At the dinners he met a woman named Allison who at first gave him the cold shoulder and then justified her inexplicable attraction to him as being part of a project to reform him, though in reality her feelings for Derek were as organic — and confusing — for her as they were for him.

As Allison grew to love Derek, and as he reciprocated, he began to trust her. Christian Picciolini, in his memoir, “White American Youth” (2017), describes the importance of empathy — how receiving it from others at a time when he felt he least deserved it was lifesaving, helping to pull him out of extremism. But the importance of empathy is primarily that it builds trust, and once you trust someone, you’ll listen to him or her. For Derek, that meant listening to Allison when she sent him dozens of studies over the course of many months showing, for example, that victims of racism had higher blood pressure, depression and heart disease. “For years Derek had been hearing about the abstract evils of racism, which he had always dismissed as empty rhetoric from his enemies on the liberal left,” Saslow writes. But Allison “made Derek begin to wonder if in fact he had been wrong in his theory that actually it was white people who were discriminated against.”

Allison also forced him to imagine the effects his ideas would have on people he cared about at New College. One weekend, while the two were driving back to school from a road trip to a state park, a relationship spat turned into a full-on ideological battle. Even though Derek had by now begun privately doubting many of his beliefs, he still clung to the idea that white nationalism was simply about defending whites, not harming nonwhites. “Your stupid theory makes no sense,” Allison finally said. “Didn’t white nationalists want to deport … minorities and uproot their lives? … Did he somehow not understand why that idea would be threatening for Rose, or Moshe or Matthew? … When the great deportation came, would Derek himself be willing to break into their homes and force them out? Or would he stand by and watch as his father and other Stormfront members did it for him?”

When Derek later confronts his dad with this same question, at least Don is honest about the implications of what he’d been advocating all along: Immigrants, Jews and blacks could “be forced to leave,” he replies, to Derek’s horror. “This country is on the verge of a reckoning.” Derek’s naïveté, on the other hand, is exasperating, and by the time he finally tells Allison, in his senior year, “I’m done. I don’t believe in it, and I’m not going to be involved,” it’s a bit anticlimactic.

It may be impossible to trace a direct connection between Derek’s beliefs and activities, on the one hand, and white nationalist violence, on the other. But it’s a discomfiting fact that at least six murderers turn out to have used Stormfront as a resource, including Dylann Roof, who logged on as “LilAryan” before killing nine African-American churchgoers in Charleston in 2015. In his 23 years as a white nationalist, Derek gave dozens of interviews and speeches, and organized a large conference promoting his ideas; his radio show influenced hundreds, possibly thousands of people.

By the time Trump became the Republican nominee for president, Derek had changed his name to Roland Derek Black, quit the movement, been disowned by some members of his family and enrolled in a Ph.D. program in medieval history at the University of Chicago. He wanted to hide — he was disgusted with himself — but Allison, ever his moral compass, insisted that he owed a debt to the country he helped divide. Like his dad and David Duke, Derek saw in Trump’s rise a validation of their efforts to popularize white nationalism, though unlike them he was aghast. “Maybe Trump wasn’t in fact a white nationalist,” Don cheers, “but he sure was good at sounding like one.”

Yet when Derek finally decides to go all out in denouncing white nationalism in an Op-Ed in The New York Times, becoming a reluctant public face of antiracism days after the election, the reader can’t help feeling that he’s getting off, and out of the movement, a little too easily. In Picciolini’s case, his wife and children left him and he sank into a suicidal depression before his rebirth as a “former”; Frank Meeink, the ex-Nazi author of “Autobiography of a Recovering Skinhead,” became addicted to heroin; and Angela King, the subject of “Meeting a Monster,” a recent biopic about her life as a skinhead, went to prison. As any reader of the Bible knows, the more dramatic the suffering, the more rewarding the redemption, and in the end, Derek Black’s transformation is too little, too late. At the conclusion of “Rising Out of Hatred,” he’s a student in a top Ph.D. program and gets to keep the girl, while his parents threaten only to take away his credit card.

“I’m part of all this, and it makes me ill,” he told Allison in the summer of 2016, a few months before Trump’s election. “I wish there was some way to not think about that.”