If you want to keep commoners calm, stop the slave talk. Like jaune stated in an earlier post, certain words cause people to go into a frenzy. Slave is one of them.
Adventurers understand noble arses exist in any realm; they take due note and avoid them. No adventurer signs up to become a slave.
Nobles, who are smart enough to understand that 'nobility' holds one to a higher standard of conduct, treat adventurers with due respect. "Due respect" for some nobles may indeed mean treating adventurers as dogs. Good realms and good nobles know better. These nobles reap the rewards of magical items, easy item repair and beast-free lands.
Ays, you just said it all. Simple cause and effect. Beat and kick the peasants long enough and eventually they'll rise up. They'll also respond positively to better treatment. It's human nature and runs throughout history.
Equality is a non-issue. Game mechanics sets the rules. Nobles can arrest and beat up adventurers - adventurers can't.
However, you can tell people what to do but you can't tell them what to think. To discourage what is not wanted, have a GM start some in-game effects such as monster/undead uprising, warehouses burning down, said noble suddenly struck down with disease along with the appropriate RP saying something to the effect of the gods having cursed the lands due to the nobility spouting heresy.
Works much better for me than an in-game OOC message telling me how to think.
