Herbert Clutter was a devout Methodist and a widely respected self-made man, who had established a successful and very prosperous farm from modest beginnings. He employed as many as 18 farmhands, and former employees reportedly admired and respected him for his fair treatment and good wages. His four children—three girls and a boy—were also widely respected in the community. The elder daughters, Eveanna and Beverly, had moved out of their parents' home and started their adult lives. The two younger children, Nancy, 16, and Kenyon, 15, were high school students living at home. Clutter's wife, Bonnie, a member of the local garden club, had been incapacitated by clinical depression and physical ailments since the births of her children, although this characterization of her has been disputed by surviving family members.
Two ex-convicts on parole from the Kansas State Penitentiary, Richard "Dick" Hickock and Perry Edward Smith, committed the robbery and murders on November 15, 1959. One of their former fellow prisoners was Floyd Wells, who had worked as a farmhand for Mr. Clutter. Wells told Hickock about a safe at the farmhouse where Herb Clutter kept large amounts of cash. Hickock soon hatched the idea to commit the robbery, leave no witnesses, and start a new life in Mexico with the cash. According to Capote, Hickock described his plan as "a cinch, the perfect score." Hickock later contacted Smith, his former cellmate, about committing the robbery with him. The information from Wells proved to be false, since Herb Clutter did not keep cash on hand, had no safe, and did all his business by check, to keep better track of transactions.
After driving across the state of Kansas on November 14, 1959, Hickock and Smith located the Clutter home and entered while the family slept. After they roused the family and discovered there was no money to be found, Smith, notoriously unstable and prone to violent acts in fits of rage, slit Herb Clutter's throat and then shot him in the head. Capote writes that Smith recounted later, "I didn't want to harm the man. I thought he was a very nice gentleman. Soft spoken. I thought so right up to the moment I cut his throat." Kenyon, Nancy, and then Mrs. Clutter were also murdered, each by a single shotgun blast to the head.
Smith claimed in his oral confession that Hickock murdered the two women. When asked to sign his confession, however, Smith refused. According to Capote, he wanted to accept responsibility for all four killings because, he said, he was "sorry for Dick's mother." Smith added, "She's a real sweet person." Hickock always maintained that Smith committed all four killings.
On the basis of a tip from Wells, who contacted the prison warden after hearing of the murders, Hickock and Smith were identified as suspects and arrested in Las Vegas on December 30, 1959. They were brought back to Kansas where they were both put on trial for the murders. Their trial took place at the county courthouse in Garden City, Kansas from March 22 to March 29, 1960. They both pleaded temporary insanity at the trial, but local GPs evaluated the accused and pronounced them sane. They were both convicted of the mass murder after the jury deliberated for only 45 minutes. Their conviction carried a mandatory death sentence during that time.
After five years on death row, Smith and Hickock were executed by hanging just after midnight on April 14, 1965, in Lansing, Kansas, at the Kansas State Penitentiary (now known as Lansing Correctional Facility). Hickock was executed first and was pronounced dead at 12:41 a.m. after hanging for nearly 20 minutes. Smith followed shortly after and was pronounced dead at 1:19 a.m. Warden Greg Seamon presided over the executions in Lansing. The gallows used in their executions now forms part of the collections of the Kansas State Historical Society.