Genghis Khan was probably not more cruel than other rulers. These were brutal times. The death toll is heavily disputed and one may argue how far Khan was directly responsible. In many cases horror stories about historic rulers are based on propaganda. Khan's political achievements are at least impressive. He conquered a vast territory, adopted technology and knowledge, he promoted education/science, reformed law and politics, provided freedom of religion, had infrastructures improved, things like that.

I wonder what the effect of plagues was on the climate, since these were absolute killers.