I agree that the Fort Pillow link seems really tenuous. There were many reasons that excessive killing of Black troops occurred at Fort Pillow. Some were military, some were the mindset of the time regarding Black soldiers and servile insurrection. It was the sort of situation in which some iron handed discipline would have to be exerted to stop things immediately after the garrison was overwhelmed. For a variety of known and proposed reasons, that didn't happen. The same would happen with another command not so far away only six days later, although there was no restraint at all in that case (Poison Spring.)
Although it has been several years since I read it, I really enjoyed Jack Hurst's work on Forrest, and Forrest is probably the most intriguing military commander to me.