Men like Simon Legree existed. More to the point, there was nothing in the legal system to prevent him from doing what he did.
Stowe portrayed a wide variety of slave owners. Many of them decent, just or even kind.
She nowhere made any claim that Legree was typical, in fact quite the opposite. But southerners latched onto what they claimed was a distortion in an attempt to discredit the entire book.
Unlike most who opine about Uncle Tom’s Cabin, I’ve actually read the book.
Black men who owned slaves also existed, and I expect in greater numbers than did the Simon Legrees.
Your comment does nothing to address the fact that Simon Legree was a piece of very successful propaganda designed to drive social change, and it did so by causing massive upheaval.
Modern Liberals used coat hangers to represent back alley abortions when they argued for legalization, and in subsequent defense of it. This was their version of "Simon Legree."
That it was true only in a very few cases makes no difference to the cause. They will topple the 99% to rationalize that other 1%.