A chorus of Black leaders, including the Rev. Al Sharpton, who eulogized Neely, demanded Penny’s prosecution. Demonstrators camped outside the office of the district attorney, Alvin Bragg, and a Los Angeles-based P.R. adviser pushed humanizing stories about Neely onto national television. On May 11, Bragg’s office charged Penny with second-degree manslaughter.

In response, Ron DeSantis, the Florida governor and soon-to-be presidential candidate, linked to a fund-raising site on his social media account that ultimately raised over $3 million for Penny’s defense from more than 66,000 donors.

The battle was set.

Rather than moving on to the outcome of the case, “Man in the Mirror” diverts. Through intensive interviews with Neely’s relatives and a friend of his, Giridharadas shows how the vortex of poverty dragged Neely’s life down to Hadean depths. In 2007, when he was 14, his adoring mother, Christie, who moved in and out of shelters with her son, was killed by her boyfriend, most likely by way of — incredibly — a chokehold. The boyfriend then dumped her body, hidden in a duffel bag, beside a Bronx highway. At the ensuing homicide trial, the boyfriend represented himself, forcing Neely, who testified, into a ghoulish confrontation with his mother’s killer.

A skilled Michael Jackson imitator, Neely had made a living busking in his teens, but eventually his life cascaded downward. With his father, in Neely’s description, “largely absent from his life,” he was soon living on the street, plagued by the familiar devils of homelessness, drug use and mental illness, in his case schizophrenia.

Giridharadas’s title is drawn from a Michael Jackson lyric as quoted at a news conference by the Neely family’s lawyers: “I’m looking at the man in the mirror and I’m asking him to change his ways.” The change the lawyers and — seemingly — the author ask of us is in our treatment of the homeless and the mentally ill.