It might be interesting to look at some major historical event, such as the American Civil War, from a systems-theoretical perspective. We wouldn’t look first at human motives or values or social practices, but social systems. We can ask why an increasingly functionally differentiated society could no longer tolerate slavery. This isn’t about morality or the hearts and minds of people, but rather about how the economic system, among other systems, could not tolerate slavery. In other words, it’s not a question of human beings becoming morally better, kinder, or more humane.
So why can’t a functionally differentiated economy tolerate slavery? Why can we easily or efficiently buy and sell material goods and labor but not human beings?
Under stratification, people belong wholly to families or social groups, and they can belong to only one. The inclusion was total–body, mind, and soul belong to the family. Members of religious orders (priests, monks, nuns) belong to the church in a similar way; thus, they cannot marry or have families. The same would apply to servants and slaves. A wealthy landowner could kill a servant, peasant, or serf without any legal consequences other than maybe paying a fine if the serf belonged to someone else. Subjects belonged to sovereigns in the same way, which is why a sovereign could or order subjects to fight in wars, and kill them if they refused.
Only in modern times do we have something like conscientious objection or passivism, and this is because people do not belong to the state. The state, which is what the political system calls itself, consists of communication, not people. So the modern state cannot force a citizen to go to war and kill people. But in wartime, the state can compel a conscientious objector to serve in a non-combatant role.