"History books" focus on leaders who have killed the greatest number of people (in the name of the nation, the people, or another sacred ideal).

Historians are complicit: the Heisenberg Principle: the observed cannot be disentangled from the observer. Political murderers are acutely conscious of historians. Historians are the mass-murderer's target market. 

Political leaders murder in order to be "remembered."  Those who murder the greatest number of human beings are those most remembered: Lenin, Stalin, Hitler, Mao, Pol Pot, etc.

Mass murderers kill in order to be written up in history books. The Heisenberg Principle. The Observed cannot be separated from the Observer.