Actually, no one ever did - the concept of organized genocide did not exist before the early 20th century - beginning with the Communist destruction of the “kulaks” of the Urkraine.
Actually, you’re wrong. The Mongols slaughtered by the millions. And as far as religion, various religions slaughtered hundreds of thousands of people before Darwin and Hitler. Just in the religious wars of the 1600s hundreds of thousands were slaughtered. Try again.
How many died in the civil war? How many died in the European conquest of the new world?
Somehow I suspect they weren't the first to have the idea. They were just the first to have the technological means and the will to use it for that.
While the term “genocide” didn’t exist the organized slaughter of people the rulers don’t like goes as far back as we can find. heck there’s two in the Bible.