> I'd say it's very hard to find any violent revolutions that improved a society.
The reign of terror in France may not have improved France, but it did scare the European aristocracy, and ended up indirectly giving a lot of political and economic rights to the general population. Aristocrats very quickly realized that they did not want a repeat of that performance in their home countries.
The Soviet Union, especially under Lenin and Stalin was no picnic, but it scared the West shitless, thus forcing it to concede rights, benefits, and legislature to labour movements, to prevent full-scale communism. Social democracy, as an institution, can credit much of its existence to the communist revolution.
People with power over you never give anything up, unless they are scared. Violent revolutions tend to make neighboring oligarchs quite scared.