Thanks @arrrados. I don't personally feel bitter about the slavery – more sadness that these things happened and still happen in some places. My mother is English, but my ancestors on her side of the family worked in the mills of Yorkshire from childhood, in harsh conditions – so many people in the countries where slavery was allowed also suffered.
Interesting too about the Byzantine soldiers, who probably developed more loyalty as a result of their more humane treatment. It's good to learn from all this.