As horrible as the Holocaust was, from a numbers standpoint alone, the transatlantic slave trade accounted for more people stolen from their homes.
Now, that's the entirety of the transatlantic slave trade. A couple of hundred years of pain and torment. But it also only counts the number of people stolen from home and sold into slavery, not the number of children born into slavery who lived their entire, and often short, lives working backbreaking labor. Well, that's what the men did. The women were raped until they were no longer physically capable of giving birth again.
Contrast the Holocaust, at most you have 10 years of horror, from the passage of the Nuremberg Laws in 1935 until the end of the war in 1945. It was traumatizing for the survivors, but the strong could move on with their lives.
Slaves who were freed at the end of the civil war were often basically re-enslaved via that fun little loophole in the 13th amendment. Corrupt judges in the south had a full racket going. They would assign court fees to and black man arrested by their corrupt police, and then sell said black man to a plantation owner as part of covering the debt.
If you thought the horrors of slavery were bad, the convict leasing and debt peonage schemes were so much worse. Literally working people to death because the plantation owners could simply pick up a new slave at the courthouse for a small fee. A smaller fee in places where the plantation owner was the judge.
Fun fact. The whole debt peonage thing did garner the attention of federal prosecutors, who thought it was fucked up. See, Debt peonage was actually outlawed by Congress in 1867. So one enterprising prosecutor actually took a bunch of assholes to court.
Those assholes argued that since the debt was fictitious, it wasn't peonage, but was actually slavery. And since congress had never actually passed a law against slavery, they wanted their slaves back. This was in 1903.