Much of the 20th century was terrible, unfathomably horrible events, I wouldn't mind a do-over.
So you'd rather go back to 40 year average lifespans, 2-3% risk of death in childbirth per birth, uncontrolled infectious disease in general, much more violent crime, and slavery/a severe racial caste system being a fundamental feature of society? All because a few sociopaths found a way to use technology to kill people more efficiently than in the past before the international community rose up and stopped basically every last one of them? Dude, life's not perfect, but get some perspective!
What are you talking about? Genocide, ethnic cleansing, human-organized famine, and the like aren't aberrations or isolated incidents, they are the direct product and natural result of the process of "modernization" that was such a fixation in the 19th and 20th century and which continues to reap horrendous consequences around the world.