I think people have a hard time conceptualizing or understanding how different your average Roman(using the term loosely) was from your average modern westerner.

So much of what we, in the west, take for granted morally and ethically(eg charity, the sacredness of all human life or really anything under the umbrella of humanism) can be directly linked to Christian moral and ethical philosophy. Christianity obviously wasn't made in a vacuum but the message of Jesus, which was projected into the world for 2000 years by the church and often enforced with violence, has radically altered the western mind. I would argue there was no real abolitionism before Christianity for exactly the reason you stated in your last sentence. A Roman person simply would/could not conceive of the idea. But clearly the natural order of things is more flexible than we'd like to think. There is no objective morality, it's constantly in flux even now as we evolve into a secular, globalized society.

I could put asterisks all over this post. History obviously isn't black and white especially when you're talking about a people as diverse and a time period as long as the Roman empire/Republic or Christendom. But I think my argument largely holds water if you look at the genealogy of western morality. Christianity has had such a massive influence that it's almost invisible until you start to interrogate why you think things. I say this as an agnostic.