I think there was a law or rule passed by the Confederates that white Union officers in charge of Negro troops were to be be killed. I don't know that it was ever put into practice -- the Union may have promised sufficient retaliation that the South didn't go through with it. I've seen reference to such Union officers being held in Confederate prisons rather than killed, but I probably couldn't find that again without a great deal of looking.
As far as Fort Pillow goes, I suspect that individual Confederate soldiers or groups of them did get out of hand, but their officers restrained them as best they could. I've seen newspaper reports of a few Union wounded being buried alive but escaping after the Confederates left. These newspaper accounts appeared within a couple of days of the battle. So I don't doubt that some bad things happened.
Part of the problem was that the black Union soldiers at Fort Pillow had been mistreating, robbing, and worse the wives and families of the Tennessee troops under Forrest. The battle gave the Confederate troops a chance to retaliate. Again, I would have to search to find where I read that.
As far as the official policy, at least in late 1864, about turning captured black Union soldiers into slaves, please see the following excerpt from an October 10, 1864 letter from Lee to Grant (Source: The Memphis Daily Appeal [then being printed in Montgomery, Alabama], November 2, 1864):
I beg to explain the policy pursued by the Confederate Government towards this class of persons, when captured by its forces.
All negroes in the military or naval services of the United States, taken by us, who are not identified as the property of citizens or residents of any of the Confederate States, are regarded as prisoners of war, being held to be proper subjects of exchange, as I recently had the honor to inform you.
No labor is extracted from such prisoners by the Confederate authorities.
Negroes who owe service or labor to citizens or residents of the Confederate States, and who, through compulsion, persuasion, or of their own accord, leave their owners, and are placed in the military or naval services of the United States, occupy a different position.
The right to the service or labor of negro slaves, in the Confederate States, is the same now as when those States were members of the Federal Union.
The constitutional relations and obligations of the Confederate Government to the owners of this species of property, are the same as those so frequently and so long recognized as appertaining to the government of the United States, with reference to the same type of persons, by its organic law.
That policy of the Confederates toward former Southern slaves who were captured in Union uniforms was confirmed by Federal General Benjamin "Beast" Butler, Commissioner of Prisoner Exchange after the war, as follows:
In case the Confederate authorities should yield to the argument...and formally notify me that their slaves captured in our uniform would be exchanged as other soldiers were, and that they were ready to return to us all our prisoners at Andersonville and elsewhere in exchange for theirs, I had determined, with the consent of the lieutenant-general [Grant], as a last resort, in order to prevent exchange, to demand that the outlawry against me should be formally reversed and apologized for before I would further negotiate the exchange of prisoners.
Thanks for the information.
IIRC some of Fielding Hurst's men were in the fort. Shortly after the battle Forrest passed a communication to the yankee command demanding that Hurst be turned over as a war criminal. The yankees said they'd investigate it or something, then refused to give him up.