I really don’t want to turn this thread into a Feducci thread, but:
Regarding the Black Ribbon:
Everyone who joined did so willingly and was presumably excited about killing people for the lulz. There’s no indication that Feducci ever forces anyone to join or catches borderline suicidal people in their moment of weakness or arranges for defectors to die to causes other than the duels they d__n well agreed to. Considering the long history the Society and its current decline, I find it very likely that everyone currently in possession of a black ribbon has taken one from someone they killed. That was the deal. Allowing a member to get out of being a target is an injustice toward everyone said member killed. If you "changed your mind", you’re welcome to stop challenging people and develop a reputation so that no one bothers you either.
"Objective" axiomatic fairness does not exist. Talking about society, I consider fairness to be, roughly speaking, the greatest good for the greatest number. Independent competitions like cycling marathons, competitive eating and dueling to the death are entirely ruled by internal compact. There’s nothing that makes competitions that favor genetic advantage (like running) more fair than competitions that favor affluence (like tennis) or vice versa, and nothing that makes either more or less fair than a coin flip (shouldn’t the "better" gentleperson win?).
Regarding slavery:
Real-life human rights aren’t arbitrary or axiomatic, they’re best practices borne out of centuries of suffering. When a douchebag combinator decides a certain right is negotiable, it is extremely unlikely he (douchebag combinators are usually "he") is correct and extremely likely he’s just ignoring mountains of evidence for personal profit. And these real-life best practices are rooted in reality: that people are sapient, independent, unique, countable, and free; that they are biologically very similar and share similar interests; that mental impairment is an exception rather than the rule; that the supernatural doesn’t exist. Out of these facts arise the "principles" of statecraft and democratic decision-making.
Fallen London is nothing like the above (hur hur). Humanity faces a dozen existential threats. Freedom is not the default; a friend you speak to can be a Snuffer who stole your friend’s face or a Fingerking who stole your friend’s everything or just have their kids held hostage and thus be compelled to express any opinion or support any stance. Information can kill you and the people you share it with. The supernatural deprioritizes industry in favor of artifice, there’s no strength in numbers, and the powerful are not beholden to the powerless in any way.
In these circumstances the benefit of doubt extends much farther than in reality. In FL, it is possible that enslaving a bunch of people is an acceptable sacrifice that ultimately serves humanity’s interests, and Londoners can’t really hold a referendum on the matter (not that real-life referenda always end well, heh). "Hey, Feducci plans to trick the devils into an alliance and betray them so we win the next war and free ALL THE SOULS, can he whip a bunch of POWs for show?" Yeah, that’ll work.
Also, a deviless is FL’s mascot, which leads me to think devils are reasonably popular and are considered to be people by a sizable percentage of the voters. These voters could view a campaign of extermination against the devils as attempted genocide and see subsequent enslavement of the combatants as just punishment.
The Princess has no benefit of doubt. She became a monster by torturing and eating people (more precisely, foods that contain people products) for fun and she had a makeover by eating people. Two cannibalisms do not make a vegetarian. And not to put too fine a point upon it, but "eyesore" slums burning down have a recent-ish real-life parallel.
Now, it’s obvious to me, especially after The Marriage of Feducci, that he isn’t really much better. They’re both monsters, and when the reckoning comes they both should be hanged on a (dimmed) street light. But it wasn’t obvious to many of the voters at the time of the election. In the electoral announcements the DTC promised temperance (her being a Society lady, this basically read as the aristocracy enjoying their '68 and honey behind closed doors and getting an extra reason to blame the poor for poverty) and the Detective overpolicing; Feducci hinted at no less than a Chain to climb.
But sure, let’s pretend there’s no difference between "you can be gods" and "you can be a steak at a royal dinner, choose rare or well-done" and that the playerbase who overwhelmingly voted Jenny didn’t want a guuuuuurl in office.