You should watch the Pierpaolo Pasolini's movie "Salò o le giornate di Gomorra". In that movie,Pasolini wanted to denounce the impunity of the people in power,so he created a story where nobles,politicians,priests,judges take some boys and girls as slaves and treat them in the worst way possible for the entire movie. Its a very hard movie to watch,and i dont know if Oda ever watched it,but the Celestial Dragons potrayal is very similar to that one. Its also similar to the potrayal of the French nobles in the manga "Rose of Versailles". Its a manga about the French Revolution,and years ago Oda said he did read it. In that manga,the French nobles are written in a similar way to the CD,there is even a scene where a noble shoot a kid in the streets just because he watched him with a bad look. So maybe these are the things where Oda took inspiration from.