Here is a direct answer to the question. There's a ton more info on the topic and a huge portion of his book is dedicated to it:
*Wasn’t the 20th century the most violent in history?
Probably not; see chapter 5, especially pp. 189–200. Historical data from past centuries are far less complete, but the existing estimates of death tolls, when calculated as a proportion of the world’s population at the time, show at least nine atrocities before the 20th century (that we know of) which may have been worse than World War II. They arose from collapsing empires, horse tribe invasions, the slave trade, and the annihilation of native peoples, with wars of religion close behind. World War I doesn’t even make the top ten.
Also, a century comprises a hundred years, not just fifty, and the second half of the 20th century was host to a Long Peace (chapter 5) and a New Peace (chapter 6) with unusually low rates of death in warfare.