When getting into these theological issues you have to ask what KIND of slavery, among other things. There's a little gem in Revelation about "slave traders" being among those who will go to Hell. That's chattel slavery. The Roman system was more like indentured servitude; these slaves were urged to purchase their freedom if at all possible.
Considering that these passages deal with acts of war and killing, and considering that there are numerious verses scanctioning slavery and regulating the practice of slavery, I don't see that this carries the weight you want it to.
I'm not sure how much worse the American system of slavery could be than one that allows the owner of a slave to beat him to death, provided the slaves lives at least 24 hours after the beating.