Do read through as many of Larry Cockerham's posts on Forrest as you can. I don't think Forrest himself knew as much! One you might try is "The Brothers Forrest". That tells a lot about Forrest's family and his background. You'll get a good idea of who he really was. One must remember that right after the war there was an effort to smear all the former Confederate commanders - Forrest was a particularly good target.
Yes, I concur that a massacre did happen - a whole lot of people ended up dead who shouldn't have. Forrest knew it would be a nasty battle - that's why his son was off with Uncle Jesse raiding a train. The men with Forrest were a green bunch and rough - Bragg had taken two well-trained veteran brigades away from Forrest and Forrest had been forced to literally beat the brush to come up with another. He had a lot of men with him he would never have looked at twice before. These men and their families had suffered a good deal from the activities of the people in the fort, both military and civilian. The notorious Fielding Hurst and his troops based out of there, and Bradford and his boys were not known for their good behavior. In fact, Hurst was likely Forrest's real target.
You had a lot of elements of visceral hatred present, going beyond racism - which was certainly present. Many of the black soldiers had been recruited from the local area, which meant many of the local men in Forrest's command knew them as well. (Forrest recognized several of the dead himself as having been through his market.) Some of the nastiest stuff was likely done during the night by outsiders - Forrest said there were locals who had sneaked into the fort to do dirty deeds and, no doubt, some of his troops sneaked away with them. But he did not order any of this, nor did he order the killing of blacks or of POWs. Forrest was noted for the humane treatment of all POWs regardless of color, although he followed Confederate policy and sent the blacks either to their masters or back to be sold. Quite a few black POWs could state that he treated them well. As cash notes, Forrest did not intend a massacre to happen, or that it should get as bad as it did BUT he did know it would be an ugly battle.
Forrest was well in the rear getting some broken ribs tended to. His horse had fallen on him - there were three shot out from under him that day - and he was about 400 yards back. As Sherman said, he couldn't see or hear what was transpiring in the fort. Chalmers was the officer in charge inside the fort, and he was the one who lost control of the men. He either could not or would not stop the killing, and the surrender flag went up and down a time or two. Forrest admitted a white flag flew at one point but he could not recognize it because his men were still under heavy fire. Finally, Chalmers cut the Union flag down with his sword. When Forrest learned of what was happening, he mounted his horse despite the doctor's warning his ribs might puncture a lung or something and kill him, and rode into the fort. Even he could not gain control until he drew both sword and pistol and promised to kill the next man who fired a shot. They knew he'd do it, too, and they stopped.
There were two Congressional investigations into the matter. One was pretty hysterical and the other much less so. Neither found sufficient evidence to charge Forrest with any crime. Forrest several times demanded to be charged and tried but never got his day in court.
"Fort Pillow" by Harry Turtledove is a good fictional account of this incident. It puts some things in perspective.
"The Diary of Belle Edmonson" is another good read for this. She was a spy for Forrest, her brother being one of his scouts, and the diary gives a very good impression of just what was going on in the vicinity of Ft Pillow at this time.