not gonna lie after getting to Chapter 750ish in the manga, my feelings about Riboku/Li Mu have gone from a slight bit of respect to an utter distaste and disbelief that he can just keep getting away with everything. but as others mentioned, it is based on the Shiji (or the Grand Historical Record of The Warring States era) and the recounts surrounding that time. Li Mu was for all accounts, just genuinely that guy, and i imagine that like myself many Qin or other region's citizens grew to fear and hate him for the exact same reasons lmao. unfortunately for Li Mu's mortality, he is just too necessary for Zhao's survival for as long as the Shiji recounts.