If you'll read the reports in the O.R., I believe that you will find that it was not at all uncommon for the Union to murder prisoners... (at least in and around Missouri.)
In 1902 the Kansas City Missouri Chapter 149, United Daughters of the Confederacy, erected a monument "To the Brave Southern Soldiers Who Fell in the Battle of Westport, October 23, 1864.”
The monument stands almost upon the exact spot where General Jo Shelby and his soldiers slept the night before the battle of Westport. Jo Shelby’s grave is close to the base of the monument. Eighty unknown soldiers, who were killed at the battle of Westport, were buried together on the field. The monument was dedicated May 30, 1902.