For me the most interesting sequence in the book is a set of two letters: one to Perry Smith (one of the killers, a schizophrenic) from his only surviving sister (his other siblings all killed themselves) while he was in prison on a previous charge, and an 'analysis' of the letter by an older prisoner who befriended him and might have been a settling influence. The first is illiterate, ignorant, small minded, innocent, and acute. The second is not quite as clever as it thinks it is, and yet wise. The interplay of the two documents, presented almost without comment, is fascinating.
The 'true crime' genre is pretty debased these days, but this book is not trashy or exploitative. The actual killing is treated with some restraint. Capote's two strengths seem to have been the ability to elicit confidences from a wide range of people, and then to render them in first rate vivid prose.