where did your number come from? Which event?

If not counting casualties in wars, I don't think US ever massively kill. War casualties can not counte for because during a war both side kill. However, GCD killed more than 10 million Chinese in China over it's 60 years rule (peace time) including 5 million during 1960's man-made farmin and 1 million 3 fan 5 fan in 1950s.