Ok let's forget centuries ago...
About non-religious fatalities, must give credit to a few nazi or communist dictator for quite some major genocides:
Joseph Stalin - 42,672,000 (+ Vladimir Lenin - 4,017,000)
Mao Zedong - 37,828,000
Adolf Hitler - 20,946,000
Chiang Kai-shek - 10,214,000
However at the internet and facebook times, to do that massive genocides is no more so cool. Terrorism is more suited to modern times.
We got terrorists of various types, let's say separatists or so... but islamic ones are the most fit for growth, because supported by population, unlike genocides supported only by a political elite or dicrator:
In conclusion, none of this would seem possible in the absence of religious devotion to an allegedly all-powerful god.