Even IF during the time that people felt free to enslave other people, because it was legal, because all the 'best' people did it, because they could afford it, because they 'needed' them, because they inherited them, because the enslaved were not 'human' anyway, or for whatever other reason, all we have to do in order to see if they felt guilt in doing so is to picture them treating animals in the same fashion.

Even 500 years ago, people may not have worried much about the wellbeing of their dogs (Even today some people of the north don't feed their dogs in the offseason but let them run free, forcing them to scavenge), their horses and their pigs but they were most certainly aware of the concept of care and protection.