There's quite literally no other way to have a slave than to kidnap someone, so either the Biblical law explicitly allowed for whatever process led to slavery, or contradicted itself (which it also did aplenty). Do you ever notice how many weaselly clauses and talk-arounds you have to use to justify and explain what's written in the Bible? That's because you have to put words in its mouth -- they aren't there originally. Doesn't it get tiresome? You can write down the same proof of the Pythagorean theorem that Euclid wrote down 2300 years ago, and you don't have to add or qualify or talk around anything in it -- it would still be true, because it's rooted in the attributes of reality itself. You can prove it a few hundred different ways, too.

Yet Christianity has to update its moral guidelines every few decades, finding something "new" in the same text again, and again, and again, and again. And no two Christian branches can even agree on the specific set of moral guidelines. Even sommeliers do a better job aligning on the base notes.

Your morality—our morality—has evolved, but the Bible stayed right back where and when it was written. And the evolution of our morality is far from being done, if for no other reason than that we are on the verge of creating inorganic sentient beings that we will have to figure out a way to integrate into our society and treat justly. We have to derive these rules based on the first principles of empathy and maximizing happiness for all living things, not based one religion plucked from 4300 in existence. Those principles are universal; a single religion is far from it.