Is it possible for Grant to decide to distribute the old Civil War military surplus to the freedmen so that they can secure their rights? Then the freedmen would be better armed than the white supremacists.
The freedmen were often armed, and usually as well as the Redeemer terrorists. But the effects of slavery were not instantly thrown off.
The freedmen had been, until a few years before, kept in a state of ignorance and helplessness. The whites had been the ruling class, organized through state and local governments, and through the Confederate government and army. It takes psychological preparation to go into a fight and be effective. This preparation can be military training, or it can be raising in a culture where combativeness is admired and encouraged. Southern culture definitely encouraged combativeness in white men. But not in blacks. I suspect that slaves who got into fights with other slaves would be punished - the owners not wanting their property damaged.
The blacks had no experience of banding together on their own, few natural leaders with any experience, no institutions for organizing to form around. The Redeemers had all of that, and rage at the thought of being governed or policed by blacks.
This is not to say that the blacks never fought back, never resisted. But they were not effective in comparison to their numbers.