Slavery might have been immoral but "brutal". I think not. They should cite some examples of George Washington's, Nellie Custis's, or Thomas Jefferson's "brutality". It didn't happen.
The slaves brought to North America were already enslaved when they were in Africa. They were lucky to escape from Africa, where slave life was far more brutal. In America, many of them lived as well as the whites. Free, comfortable housing. Free food. Guaranteed work. Still, slavery is an evil institution, and we are still paying the price.