I just finished Noah Andre Trudeau's Like Men Of War, which is a history of black soldiers in the Civil War. It obviously pays considerable attention to Nathan Bedford Forrest's massacre of black troops at Fort Pillow. But, because Trudeau's book is so sweeping, it actually (almost unwittingly) provides context to Forrest's war crimes. Let's consider this letter from a white Lieutenant of colored troops commenting after the fight at Fort Blakey in Alabama:
The rebel line of skirmishers seeing us comung up fell back int their works. As soon as our niggers caught sight of the retreating figures of the rebs, the very devil could not hold them. Their eyes glittered like serpents and with yells & howls like hungry wolves, they rushed to the rebel work...The rebs were panic-struck, numbers of them jumping into the river and were drowned in attempting to cross, or were shot while swimming. Still others threw down their arms and ran for their lives over to the white troops on our left to give themsleves up to save being butchered by our niggers. The niggers did not take a prisoner. They killed all they took to a man.
To my mind, there was the regular Civil War, between North and South. And then there was this second Civil War between slave and master, or freedman and slave-master. The Civil War, which we think about, was brother against brother--sometimes literally. It's McClellan wanting to prosecute a "gentle" war. It's prisoner exchanges, and parole. Then there is the second Civil War, in which you have two people who, in the old phrasing, "just don't like each other very much."
Many black soldiers were essentially impressed into the Army. Others went to strike a blow for freedom. And still others went to preserve the Union. And then there was that final group--those looking to settle old scores. In truth, none of these groups were mutually exclusive from the other. But I think, when we talk about black soldiers in the Civil War, the noble chivalrous sheen (think Glory) doesn't account for that oldest of human emotions--vengeance.