This is a  seriously wrong both fact-wise, and as a line of reasoning. Prior to the European expansion in the XV century major imperial conquests did not usually result in mass exterminations. The old type of empires were "soft-touch", aiming at some loose integration and exacting tribute without destroying economy and life in general. The kind of utter devastation that the Europeans brought to the Americas, Africa and Australia was unknown in earlier epochs. So, implying that Mongols' effectiveness was due to the "kill everyone" doctrine is deeply ahistorical.