I think you misunderstood my point. I'm criticizing the author's writing choice, not saying slavery shouldn't exist in the setting or judging the era by modern standards.
We already have a villain protagonist, but the author still felt the need to sanitize her buying a slave by introducing an "ethical" slave trader whose slaves are well treated and trained.
It comes across like the author wanted the protagonist to own a slave without the moral discomfort that usually comes with it. Better stories either lean into those uncomfortable implications or use them as a plot point later.
Instead, the MC just buys a slave with almost no ethical or narrative friction. That's what I find iffy.

Some elements are still iffy like why put in slaves and talk about ethical slavery? I mean, you took away their agency. What about those that can't be trained or sold? It's kinda funny they're ok with making the protagonist have dubious morals and have children being beheaded. But wanted to avoid uncomfortable questions with slave ownership(then just don't have her buy a slave geez).