You think that Bqi Qi might of killed much less than what was stated, yet how do you know Genghis Khan did not kill much less? How does your logic for Bai Qi not apply to Genghis?
The total number of soldiers and prisoners Bai Qi killed was 1 million and 600 thousand,according to SiMaqian's ShiJi.The real amount is probably much smaller.And there is no evidence that Baiqi had ever killed plebeians, although he had many chances.Xiang Yu killed many plebeians, and he was a rare inhumane butcher in china history,but he didn't massacre every time he occupied a city while Genghis Khan did.In fact,Xiang Yu had a long way to catch up with Genghis Khan.
Xiang Yu killed as often and as ruthless as Ying Bu, or any other generals of his time. If you bother reading Shiji, you will notice that Xiang Yu is not the only one who ‘屠之.’ And how do you know Genghis massacre every time he occupied a city? Do you have any proof for making that claim?
You obviously never read about the time of fragmentation, or for that matter, Three Kingdom, or N/S Dynasty, where population dropped in such drastic measure that it is hard to imagine anything but massacres and constant warfare. And these involves as many foreign incursion as there were civil wars.
In Chinese history most butchers were nomad, such as Manchu, Haolianbobo.
Men such as 冉闵,黄巢, 朱元璋，朱棣，张献忠， are not shy of putting their people to the sword.
To generalize that most butchers were nomads is nonsensical.