What are you talking about, Old Testament, New Testament? You think it matters? It's the word that matters. The word is the key to our survival as a species on this earthly plane. It's not that complicated. In fact, it's pretty simple. You overthink things and become unhappy and cynical because you think you don't believe in God, I guess. That's your path and it's alright by me, but it's a long way around. Slavery in the West didn't just end by itself, you know. It ended because of the militarily-backed moral force of Judeo-Christian nations who know slavery is wrong where most of the rest of the world had from time immemorial
and still do think slavery is part of being human. And as far as I know, there is no text in either old or new testament (though I am no biblical scholar by any stretch) that explicitly instructs us to reject slavery. But 50 generations of Judeo-Christian upbringing brought about its end. You are a product of that, I am, my declared-atheist dad is -- it is what it is. You can flatter yourself that the inner goodness of man would have done the same without Judeo-Christian rules if you want. I hold no such illusion.
Marrying girls at the age of nine, subjecting women to subhuman status, having slaves, lust involving adults and children, human sacrifice, disrespect for human life, cruelty, and ENVY (probably our biggest and worst immediate enemy) -- you really think these things disappeared from the "norm" by themselves?
I know they didn't.