To twist the famous Spiderman quote, sometimes with great success come people who think you should shoulder great responsibility too.
On November 16, 1959 Truman Capote came across a news item in the inner pages of New York Times about the Clutter family that was shot dead in a Kansas village. He was already a successful novelist and a short story writer. He had a good life, he was famous and infamous, he moved in important circles. Like a compulsive writer, he was looking for a new subject. This item appealed to him, and he decided that he would write a book on it.
He wanted to write a true account, a novel that was like a newspaper article, meticulous and factual rather than fiction. The best way to go about it was to go down to Kansas and research for his book material there. He recruited the assistance of his longtime friend, Nelle Harper Lee.
Nelle was able to help him gain a foothold into the Kansas community. He wormed his way into people's minds to gain their confidence, elicit information that was necessary for the book. He even bribed, charmed and cajoled his way around to be able to get up, close and personal with the two killers, Smith and Hikock. He used tact and tricks, lied and twisted facts to get his way. He plied the prisoner with books, and even promises of help with their case.
He even arranged for a lawyer for the killers which made them look upon him as a savior. They start believing that he is out to help them and opened up to him. It was a victory for Capote. To be able to look into the mind of killers first hand. He had the background of the victims in place, thanks to Harper Lee and her help. The investigations were revealed to Capote thanks to being close to the Police.
Back in New York, in the literary circles, he was able to generate curiosity about the book even without having written a word, such was his way with people. He was able to sell the idea of the book. When he did write the book, it lived up to the hype. In fact, he created a new genre, the factual fiction. Capote had the right ingredient to make it work, the working of the mind of a criminal. The book was serialized in New Yorker, but was yet to be completed.
Until the case concluded either way, in pardon or in execution, he could not complete the book. He was looking for closure. Admittedly, a death-sentence would provide the required thrill to the end. A pardon would water it down. The tussle here was between being an opportunist, and being humane towards the killers. It is implied in the movie, that through inaction, failure to provide the killers with additional legal help, he actually was instrumental in getting the killers executed.
As a professional writer, Capote's motive was to get as deep into the scene of the crime as possible, as deep into the minds of the criminals as he could. Whether it automatically pegs on him the responsibility to help the criminals or not is debatable. The fact is, he could not have written a novel as compelling as 'In Cold Blood' unless he had broken the barriers of niceties.
As for the other faults that the movie listed, showing off, partying, promoting himself shamelessly, well, every person has these quirks which make them interesting and tolerable. It is the faultless people who are so boring and intolerable.