However, as others have said, superior technological and sociological advances are no justification for slavery. I feel that any rational moral justification for slavery is intellectually dishonest.
That being said, many of the cultures in Africa where slaves came from engaged in slavery often with barbaric treatment toward their slaves. The slaves did not come from peaceful societies and essentially the same fate could of befell many of them even if they were not abducted by Europeans. Again this is not a justification for the treatment the slaves received, just the reality of the situation in West Africa in the 16th and 17th centuries.