I have not seen the documentary, so I cannot comment on how it handles the territorial conflict. But, in regards to your posted letter, your simplification of the historiography poses problems. There are not only two sides to the story. In fact, historians today deal with multiple viewpoints and interpretations of what happened. Portraying the story of the Kansas-Missouri conflict, or the Civil War for that matter, as an "either-or" or "us versus them" situation fails to appreciate the complexity of the period as well as promotes outdated Lost Cause ideas.
As a result, "balance" is not achieved by finding, for example, one person who criticizes Quantrill and one person who defends him. Instead, balance can be better attained through covering many different topics and elements of the territorial struggle.