To add to this, the reason why the British and American system of slavery, was so evil and morally bankrupt, was because it institutionalized the racism.
We know that throughout human history, there had always been slaves. Whether they were from war conquests, prisoners, or what not. And maybe, some of them eventually got their freedoms, after serving for a term. Basically, they came and went, and no one thought anything of it.
But, what truly made the British and American system of slavery so evil, was that it became codified into laws. And written on papers, and enforced by the military, and police. And financed by the banks. This was truly, predatory capitalism, of the highest order. Thus, slavery became institutionalized.
This elevated slavery, from something that unfortunately happened to random people like prisoners, to completely enslaving, subjugating, humiliating, and genociding, a specific race of people, purely based on the color of their skin. And all this, happened in a democracy.