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 slav