If greeks thought citizens should respect the rights of others, enemies, and minorities, why they deemed slavery as natural and necessary? Also, I don't think barbarians were selfish. People who lived in foraging societies shared their food with the community. Also, the power imbalance within the community was minimal.
Elites always tell us to be good hard-working responsible citizens. What do we get in return for being one? Even in more progressed and developed countries, common citizens will be never seen as equals to those elites.