to the last century, whether meaning all of the 20th century, or the past 100 years from 1920 to present, must include the killings from the wars I mentioned, e.g. African and ME nations fighting for independence from colonial rulers; chronic war and terrorism in Israel; India's struggle for independence; Hindu vs Muslim in India; lynchings in the US.
Ideologies of tribalism and social Darwinism were behind a lot of those killings. Emphasizing those ideologies as the true biological nature of human beings, based on our animal relatives, encourages that kind of behavior and thinking at the expense of other qualities that are equally part of our biological inheritance through evolution.
That's my main point because I am disturbed by how often lately I am coming across this idea in lop-sided "studies" and conversations with people who buy into the notion that hostility and distrust of "the other" is our true nature. That notion leads people to conclude that other natural human qualities like negotiation, compassion, peace, cooperation, and mutually beneficial interactions across social and cultural boundaries are artificially imposed on people by civilization, abd that they can be easily discarded, and should be.
RE: the killings of the Middle Ages. That term implies European history from the fall of Rome to early modern times. There were many other cultures around the world existing in the same time period. Some emphasized peace and the value of life and others, not so much.