I would measure the brutality of the Worlds Wars by the number of deaths. Of course all statistics of military, civilian, and prisoner of war casualties are estimates. For World War I these estimates range from 9 million to over 15 million. For World War II, estimates range from 50 million to over 80 million.
It is almost unbelievable to think that if 80 million people died during the course of World War II that equates to, at that time, almost 3% of the world’s population. One has to wonder why it is important which World War was more brutal. The world obviously didn’t learn from World War I and ended up fighting a second world war.