This is a very interesting question to ask, as the tone of this book seems explicitly related to what Capote tries to do with it. Although the author set himself the task of using the raw material of this case to write a non-fiction fiction novel, it is clear that what we are presented with seems to be a fascinating study of one real life case where what the author includes and doesn't include is just as interesting as the details of the case itself.
Capote's tone in the novel strives to be objective, but he cannot help but let his sympathy towards the criminals emerge. In addition, what is interesting is the way that Capote clearly communicates several other themes that are very important to him such as the fragility of the American Dream and the way that it can so easily be shattered abruptly, just as Herb Clutter's life was so quickly brought to an end.
Because of this, perhaps the novel could have been written in a much more emotive tone that would have captured the feelings of such characters as Herb Clutter and his family. However, if we change the tone, we need to remember that we change the mechanics of this novel, and as a result, perhaps it is better that we leave it as it is.