As a rule, when people read fiction from another time period, they expect the culture / thinking style from that period. I can't remember how many books I've read based in medieval Europe, where the hero talks about the rights of man, individuality, democracy etc. I usually throw such books away, as it shows the author hasn't done their research.
Second thing is, you assume that all people in ancient times were cruel, and supported their respective social evils. So for example, all Romans loved to see people being eaten by lions. This is not true. Just like in modern times we have racism, religious wars, sectarian violence, but in actuality very few people actively support extremist groups. 300-500 years from now, today will look like the dark ages.
To answer your specific question, people have their own moral compass, which they follow, no matter what the society believes. So you could have your hero supporting slavery, but showing a random act of kindness to a slave. Or he could hear the story of how a slave was captured, and change his views on slavery. Or he could accept slavery, but not have personal slaves, as it doesn't agree with his own moral compass. The characters don't need to be black or white.
In short, don't force modern, western enlightenment views on your characters. But that doesn't mean they have to be barbaric "Ooga Ooga, me steal woman" type characters. Your characters can be nice people, trying to survive in a unjust society, and doing their small bit to help others.