(Transferring prior answer on locked thread)
Forrest was quite a character…
He was, before the war, one of the leading slave traders in the country, a millionaire, he used slave trading to pull himself and his brothers up from a hillbilly birth.
He could barely read or write. He was fearless and quick to escalate disputes to violence.
He was respected (perhaps even feared) by his slaves, but there is no indication that he ever mistreated them and (like most “poor white trash”) willingly worked with them.
He opposed secession, thought the South could not exist economically without the North, resigned his elected position as City Councilman in Memphis in part to avoid the secession controversy. His business was heavily in debt to Northern banks. He was also trying to diversify away from slave trading, turning the business over to his younger brothers.
He enlisted as a private after Ft Sumter but quit and formed his own cavalry unit with his own funds. Like many hill folk , he found the Southern army controlled by the Tidewater elite and their West Point graduates. (Many hill people, even in the deep south remained loyal to the Union but not Forrest.)
He first gained notoriety when he refused to surrender his unit at Ft Donaldson to General Grant. Instead he led his men through a partially frozen river at night past the Union lines. His unit gained the respect of the South and the anger of the North with numerous raids against Union supply lines and installations, defeating every attempt to destroy him. He was not as effective in major battles, however, probably due to the small size of his cavalry as compared to the cavalry in the east under J.E.B. Stuart.
It is debatable whether he ordered the death of any African-American prisoner - even though the Confederate Congress had ordered such punishment. Forrest actually preferred to attack black garrisoned forts, feeling (correctly) that the lightly trained blacks would surrender.
His troops were involved in two war crimes during the Civil War, the massacre at Ft. Pillow and the much smaller massacre at Selma. In both cases prisoners were murdered, in the former mostly black, in the latter (at the very end of the war) white oficers. Never directly implicated, Forrest was never even held on charges. To do so would require holding Robert E Lee as well as Lee commanded the troops that murdered black prisoners during the Crater Battle during the seige of Petersburg, Virginia.
Never captured, Forrest was the last major Confederate commander to surrender (excluding Texas). He quickly endorsed loyalty to the Union and began to rearrange his finances. Northern generals were relieved, they had assumed that if the South opened a guerilla war, Forrest would lead it.
He did not form the Ku Klux Klan. Still basically illiterate after the war, Forrest couldn’t even conceive of the sophmoric organization and rules of this secret society. He probably wasn’t even the first Confederate General to endorse the group and its immediate tactics - intimidating black voters.
But he quickly joined and became its leader. Under Forrest, the Klan started to reconcile Confederates with hill people loyal to the Union but opposed to equal rights. He disbanded the organization when it became clear that the Klan would perpetuate (and worsen) Union occupation of the South. Being a secret society, the Klan also quickly got out of control of the college educated officers that created it.
Forrest struggled to pay his debts after the war and engaged in a number of failed ventures. He alienanted his racist friends by denouncing white southerners as lazy and called for Asian immigration into the South. He also made a speech calling for reconciliation between blacks and whites. He died in 1877 of typhus, not yet 60, contracted by working alongside convict laborers in the fields of a farm he managed.
The Klan did not go away but extended its reach into fertile grounds in the border and Northern states (especially Indiana). As the South stagnated (and blacks agitated for equal rights), the Klan became much more violent than under Forrest. Forrest’s last years were quickly forgotten and he became a symbol of Southern resistance, hatred and intolerance.