It is a problem with revolutions, during the initial period after the battles are won, the victors tend to act just as badly as those they overthrew. The French Revolution is an excel;lent example. As is the Russian Revolution.
Atrocity begets atrocity, vengeance begets vengeance.
This is also true in regard to wars between countries.
It is a truism that the peace established after the First World War ended laid the groundwork for the environment that allowed those who seized power in Germany and Italy just prior to the Second World War to do so. The peace treaty was explicitly punitive. It was set up to place Germany in an inferior trade position, to cripple their economy, to make them suffer. To insure that they remained an enemy, just one that they believed to be incapable of posing a viable threat.
The peace treaty following the Second World War shows that someone actually learned from that, to an extent.
Anyway, in the world of this Manga, the Nobility abused their position, the Commoners' finally said, "Enough!" and overthrew them, but then proceeded to commit Crimes against Humanity themselves. Rinse & Repeat.