For those unfamiliar with it, Truman Capote's In Cold Blood documents the brutal 1959 slaying of the Clutter family, a well-respected and successful farming family in Holcomb, Kansas. Capote claimed the book defined a new genre, the "nonfiction novel." But, perhaps unsurprisingly, given that Capote mentally categorized it with the word "novel", many have raised doubts about Capote's accuracy and objectivity.

Amy Standen, writing in Salon:

After "In Cold Blood" was published, [Truman] Capote's friends and detractors (and he had plenty of both) would remark on the parallels between the author and Perry Smith, the more sensitive and guilt-ridden of the two killers. Possibly, Capote felt a physical kinship to Smith... More likely he simply understood that what separated him from Smith, more than anything, was luck.

Wikipedia, on Capote's omissions:

One of these critics was J. J. Maloney (d. 1999), a convicted murderer who upon release in 1972 became an investigative reporter for the Kansas City Star (he was nominated for the Pulitzer Prize five times) and later a Web publisher (he launched www.crimemagazine.com). Maloney gave much thought in his crime writing to criminal intent. He surmised that Smith and Hickock had begun a homosexual relationship in prison, and that at the Clutter house Smith had "exploded" when he found Hickcock intent on raping the bound Nancy Clutter. At this point, in Maloney's view, Smith challenged Hickock to kill the family, beginning with the father, as Hickock had previously bragged he would do when talking about the future robbery in prison (though Smith expected Hickock to renege on his threat and thereby show himself humiliated, at least to the two of them, as an unmanly coward). When Hickock did back down, Smith killed the whole family, to show Hickock that he, Smith, the passive partner in their relationship, was even manlier than his active-role sex partner. Maloney felt Capote missed all of this; or rather, he believed Capote had ulterior motives for covering the true account up.

As I understand it, the identity (much less motives) of the murderers were not known when Capote started his investigation. Perhaps upon discovering a situation similar to his own, Capote (no prude otherwise) felt that these particular sexual dynamics needed to be kept under wraps, withheld from the public eye.

As the Salon quote above indicates, there was little dispute that, judging from Capote's reporting, Smith seems to have been given an extremely positive portrayal. Maloney delves into this topic further: how, even though Capote ultimately concludes that Smith murdered the four victims, Capote strangely gives Smith a positive, sympathetic portrayal, while seeming to place blame for Smith's actions on his partner, Hickock, who, in fact, seems to have murdered no-one.

Further odd behavior accompanied Smith and Hickock's executions:

It was Capote who told Plimpton that, shortly before Smith was taken out to be hanged, Smith kissed Capote on the cheek. It's the only time I've ever heard of a condemned man doing such a thing.

Whether or not they had a physical relationship (it's not necessary to the premise of this piece), there's no doubt Capote fell in love with Smith during the five years he visited with him on death row. His book reeks of it.

And that is why Hickock was portrayed so mercilessly in the book — not only as a child rapist, but down to the point of saying that Hickock swerved his car in an effort to hit a dog walking down the road. Capote — in the tortured logic peculiar to those in love — blamed Hickock for Smith being on death row.

That is why, on the dreary, rain-swept night of April 14, 1965, Capote stood stoically and watched as Richard Hickock was hanged. But when it came time for Perry Smith to be hanged, Capote couldn't watch.

Recounting the crucial moment in the crime, in an act of apparent mind reading (and perhaps projection) Capote tells the reader that what triggered Smith (portrayed as otherwise sensitive and caring) to go into a murderous rage — a sudden, unconscious, and otherwise-unexplained burst of anger at "them", elements of society at fault for the failures in his life.

Yet what was actually happening at that moment Smith decided to kill, according to Capote's own account, was that Smith had discovered his partner attempting to rape the Clutters' daughter. That would seem a rather strange moment to suddenly become angry at society, wouldn't it? Thus Maloney instead posits jealousy, noting Smith's discomfort on other occasions where Hickock had sex with a woman.

"Capote romantically theorizes that Smith prevented the seduction of Nancy Clutter on moral grounds." Again, 'moral grounds' — preserving her honor, allegedly — would be strange motives indeed to impute for the murder of a girl and her entire family, no?

Whether Capote's bias was conscious or unconscious (Maloney gives evidence of conscious deception) I suspect Capote may have been unwilling to depict (or perhaps even admit) that a much-respected Kansas family could have been murdered as a side effect of a bisexual-gay relationship gone horribly awry, or admit that Smith (with whom he had at least emotionally bonded), was, at the end of the day, responsible for his own actions and crimes.