I think with regards to slavery you are bringing in modern views of humanity. I think it was the modern African chattel slavery practiced by the West that introduced the concept that slaves were less than human.
Prior to this, slavery was just considered something unfortunate that happened to a person. A person could become a slave because of debts. A person could become a slave if they lost a battle or their city was conquered. For sure, it sucked, and they did not have any rights, but again, there wasn't really a concept of human rights. In addition, there was no real concept of equality. These were very hierarchical societies. Thus, a slave, I think, would have been considered human, but at the lowest rung of the social hierarchy. In addition, there was no real ethnic component to slavery. There were Greek slaves and Greek masters. Slaves also were not considered intellectually inferior - see for example the Greek physicians and tutors that Romans would have as slaves.
Ironically, it was the push for equality and human rights that made African slavery so toxic. In the past, without concepts of human rights or equality, slavery was just what happened to people who were really unlucky somehow. However, with the concepts of equality and human rights, people realized that slavery was incompatible with that. However, they still wanted to enslave Africans. The way they dealt with this dilemma was to say that Africans were not really human, which magnified the damage caused by slavery a 100 fold, since that horrible idea persisted even after slavery ended in the West.