Fort Pillow, Andersonville?
There were claims and counter-claims about the conduct of soldiers on each side of the battle. The truth may never be fully known.
To my knowledge Generals Grant and Sherman never found evidence that justified punitive action after the war when victorious Union forces were in total control of everything including their enemies and the judiciary.
That said, I would be surprised if you were to provide me with gas-tight evidence showing no Confederate soldier in four years of war ever treated an enemy unkindly.
There is a dark side of combat that finds its way into even the most wholesome causes. https://www.youtube.com/watch?v=I7dmcHoODZI