The Mongols only killed those who would not submit, and were largely indifferent to religion among their subjects. They did not kill simply because of who or what you were, as the Nazis did. Their violence was a tactic deployed to ensure control, not a means toward the end- extermination.
The religious wars of Europe lasted hundreds of years. No one can claim that anywhere near six million were killed in a single decade.
My point is some people are killers. They hate humanity and they love to kill. Whenever there is some sort of crisis where violent groups or people can ascend to power, many times the most violent people or groups do ascend to power. And they start their killing. They might have some sort of ideology (communism, Nazism, fascism, humanism, etc.) that they claim guides them, but basically they like to kill. Because they’re psychopaths first, ideologues second.