"IF" Forrest was the First Grand Wizard of the KKK (Which is very highly debatable) then they had existed for some time without. Forrest was not in on the forming of the KKK. In its early years, it actually had some Black members.
When Forrest saw it had deviated from its goal & was becoming violent he "Dis-Banded" it.

After the war, Forrest was tried by both a Military court (USA) & Civilian Court (USA) on charges from Fort Pillow. Both courts returned a "NOT GUILTY" verdict. "IF" indeed anyone was shot while attempting to surrender it was on an individual basis, not a unit as such. Fort Pillow had been offered terms of surrender which they refused. As they were leaving the Fort to go down by the river where a steamboat was to pick them up & where incidentally they had arms & ammunition stacked They Did Not strike the flag in surrender. Forrest's men had been given orders to fire on anyone until the flag came down. After Forrest gained the fort, HE gave the orders to Strike the flag to stop the firing & thus un-necessary loss of any more lives.

"There Was No Massacre", it was Warfare. Everyone killed was a "Soldier". You want to talk about Massacre, talk about Sherman in his march across Georgia & then up through S Carolina where countless "Civilians were Murdered".

Miller/TN
I Didn't Say Everything I Said, Yogi Berra