There isn't a person in the world who isn't descended from slaves across the majority of their family tree. Before it became common to use beasts of burden in farming and before the metal needed for that became easy to acquire, slavery in various forms was a brutal economic necessity across the world. People knew it was wrong but the alternative was widespread starvation. Once there was enough metal to hold together tack for beasts of burden every form of slavery started being abolished country by country, continent by continent.
Only in the Americas was it about race and there only because serfdom was being abolished across Europe as the colonies were founded so slaves had to be imported from elsewhere. Because slavery was also being abolished in Asia at the time the only source market was from Africa. By the time slaves were shipped to the American colonies it was about profit motives not about starvation so of course the institution was doomed in the Americas as it was everywhere else. The founding fathers of the United States knew this but they also knew they could not at that time pull off a ban that would pull the support of several colonies and result in the short of disunity seen in Europe with endless wars. Waiting a century to ban slavery and that with a large war was the price paid by the founders, knowing the result would be fewer wars than occur in Europe to this day.
Knowing the facts of history is vastly different from getting it on any emotional level.