Having listened to some history (some who don't like him would say pseudo-history) lectures by Dan Carlin on the whole Mongol thing, my takeaway was that the reason for all the murder is the classic dehumanisation approach.
Mongols did not consider the people they conquered to be far above cattle. When they took over cities they staged systematic executions where they would literally corral a hundred thousand people together, and have a thousand of their soldiers armed with axes be told "go collect 100 right ears to prove that you've killed your quota of people". As far as I can tell the way to get humans to do that to other humans seems, throughout history, to be to convince the murderers that the people they're killing aren't really human, they're just some sort of weaker other-species.
Mongols did not consider the people they conquered to be far above cattle. When they took over cities they staged systematic executions where they would literally corral a hundred thousand people together, and have a thousand of their soldiers armed with axes be told "go collect 100 right ears to prove that you've killed your quota of people". As far as I can tell the way to get humans to do that to other humans seems, throughout history, to be to convince the murderers that the people they're killing aren't really human, they're just some sort of weaker other-species.