The world population in 1940 was about 2.3 billion, so WWII killed around 2% of everyone alive. Compare with the An Lushan Rebellion, which killed somewhere between 13-36 million people at a time when the world population was only about 200-300 million, killing 4-18% of everyone alive.

The 36 million is most likely untrue though, we're probably closer to the 13 million one (which would still be twice as much as WWII when it comes to % of global population)