Methinks at the bottom of it is another question: Suppose you have a knife and you are standing behind a known mass murderer. Killing him would save a million lives. What would the Buddha do?
Is there any sutta on such a problem? (I’m only aware of a story on suicide when unbearably terminally ill.) It is called the Trolley Problem: Trolley problem - Wikipedia
There are variants of the situation: E.g. you might have the opportunity to just press a remote control button to kill the mass murderer. – This can make a difference for “normal” people. Here is an article about it. The result is a bit disconcerting 
Monks similar to psychopaths?!