the 20th century has 0 monopoly on war and the atrocities that come as a result.
What's been done in the 20th century, in fact, barely scratch the surface of depravity when compared to notable horrors of the past.
The only difference is the mass media being able to shine such a light on it.
I don't disagree that the 20th century has a horrific record.
But to disregard the same happening all throughout history is to deny the darker side of humanity - and tossing out historical evidence of a larger problem, choosing to only focus on a subset artificially reduces the ability to come to a conclusion that is truly representative of the problem. Thus, precluding any possibility to find a solution.
see: https://en.wikipedia.org/wiki/Genocides_in_history