In 1972, on “The Tonight Show,” Johnny Carson asked Truman Capote about capital punishment. Capote had written, in unsettling detail, about the hanging of two killers, Dale Hickock and Perry Smith. Carson said, of the death penalty, “As long as the people don’t have to see it, they seem to be all for it”; if executions occurred “in the public square,” Americans might stop doing them. Capote wasn’t so sure. His hands laced together professorially, he murmured, in his baby-talk drawl, “Human nature is so peculiar that, really, millions of people would watch it and get some sort of vicarious sensation.”

Capote’s book “In Cold Blood,” which began, in 1965, as a four-part series for this magazine, was preoccupied both by the peculiarity of human nature and by the vicarious sensations that peculiarity can arouse. Perusing the Times in 1959, Capote noticed a story, “Wealthy Farmer, 3 of Family Slain,” about the apparently random murder of Herb and Bonnie Clutter and their two teen-age children in Holcomb, Kansas. Capote set off for the high plains.

He was fascinated, as he later explained, by “the homicidal mentality,” and felt confident that readers would share his interest. Lurid tales of real-life murders were a staple of pulp magazines. But Capote wanted to elevate this tawdry genre into art, using careful reporting, subtle characterization, and (in his own immodest explanation) his “20/20 eye for visual detail.” He announced (with further immodesty) that “In Cold Blood” marked the advent of a new form, the “nonfiction novel,” which employed “the techniques of fictional art but was nevertheless immaculately factual.”

As boasts go, this one was ill-judged. By his own admission, Capote had been inspired by Lillian Ross’s 1952 account of the making of a Hollywood movie, “Picture,” which also originated in The New Yorker and exemplified the kind of narrative reportage that he now claimed to be pioneering. Worse, “In Cold Blood” wasn’t “immaculately factual.” It included not just imagined dialogue but invented scenes. One problem was that Capote disdained notebooks and tape recorders, relying instead on his memory, which he insisted was also 20/20—or close to it. “Sometimes he said he had ninety-six-per-cent total recall, and sometimes he said he had ninety-four-per-cent total recall,” George Plimpton joked. “He could recall everything, but he could never remember what percentage recall he had.”

Capote’s transgressions were serious, but there is no denying the awesome influence of “In Cold Blood,” which encouraged both readers and writers to rethink the possibilities of nonfiction. Capote hadn’t visited Kansas before arriving in Holcomb, and his book is suffused with a rich sense of place: the merciless weather, the vernacular music of local voices (“Time was wasn’t anybody here wasn’t my kin”). With the structural precision of a suspense novelist, he crosscuts between the Clutters during their last days and the ex-cons who will rob their home. Nancy, aged sixteen, writes in her diary that final night. Capote quotes the entry—it is moving in its banality—but also notes that Nancy changes her handwriting throughout the diary, “slanting it to the right or to the left” as she tries to settle on what kind of person to be.

The most startling aspect of “In Cold Blood” is its nuanced portrait of the criminals. Covering the case over five years, he came to know both men with a discomfiting intimacy, particularly Smith, the more soulful of the two. Capote keeps returning to Smith’s strange physique—his bulky upper body, honed by weight lifting, atop stunted legs and feet so tiny they could have “fitted into a delicate lady’s dancing slippers.” Like Dostoyevsky, Capote doesn’t portray his killers as demonic ciphers, instead capturing their messy complexity. The real horror is that the murderers are so thoroughly human.

A tale starting with one set of violent deaths ends with another, in the execution chamber. Capote finds little vindication there. “Nice to see you,” Hickock says to the spectators, as if “greeting guests at his own funeral.” He seems sad that no Clutter relatives are present, as though “the protocol surrounding this ritual of vengeance was not being properly observed.”

Alvin Dewey, the Kansas lawman who apprehended the killers, does attend. He recalls the first time he saw Smith, on a police-station chair, his little feet “not quite brushing the floor.” As Smith’s body jerks on the rope, Dewey sees those “same childish feet, tilted, dangling.” ♦

An unspeakable crime in the heartland.