Maybe the word "winningest" is misleading. The "peace and alleged prosperity" after Genghis Kahn's hoards murdered about 11% of the known world's population happened because the Mongol empire stopped killing civilians for a while. What did it take to be a "winning" general in the 14th century? Tactics? All you needed was a hoard of maniacs who were willing to terrorize civilians with unspeakable atrocities. Then again maybe it is similar to WW2.
11%? Not hardly. You've been reading 19th century propaganda if you believe that nonsense. Genghis's tactics were so good both the Germans
and the Russians studied them prior to WWII. The Russians used them to the best effect prior to the launch of Operation Typhoon.
Genghis's army never mustered more than 100,000 people and he crushed everyone who went against him. Yes, they killed a lot, but unlike the European kingdoms of the time, torture was not used. That was anathema to them.
When Kublai was the Great Khan he modified the Chinese legal code and reduced the execution rate by an astronomical amount....in near 30 years of rule they executed only about 2700 if my memory serves. Europe killed WAY more than that.
Like I said, your knowledge of Genghis is primitive at best, and warped by your terrible source material.