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.
My suspicion is the rise of darwinism (who was a brit) intersected with the rise of colonialism. Colonizers may not have been too accepting of the whole "everyone comes from adam and eve" narrative when they met locals they deemed to be savages and inferior. Not that I am saying darwinism is the cause,just that their greed and arrogance thrived on any ideology that can be abused to help them with their goals.