Both World Wars (15 million in the First and 60 million in Europe's Second civil disturbance) beat each other out for the "Guinness" historical record for highest death count. Of course, with Mao engineering the death of 40 to 70 million people, Stalin engineering 9 million corpses, and Pol Pots 1 million in Cambodia one could argue that Communism or Socialism is the history's worst genocidal murderer.
No matter the culprit, that's a total of about 130 million people that died in the twentieth century without religion behind it or God instigating a thing. The "Age of Ideology" was horribly brutal to humanity. It was an age where idiots professed to engineer systems of governance that "perfected" society. Perhaps we have learned as a people that imperfect human beings will always dissapoint the "perfect" system. However, many in this world still long for that next "-ism" to create utopia, don't they?
"God" is a default answer that lacks insight.