While I'll not disagree I your selfdefense argument, you do know that those depraved people ARE the authorities, no? Nobles and Moguls for which 90+% of the population would be working for, in a post-medival society like this one. The "victims" aren't slavers, they're thugs and are likelly to be slaves themselves. As for the use of lethal force that's just the mangaka going full shonen, a bribery attempt would make more sense - "No one gets hurt and you make some proffit" type of deal .