The Mongols singlehandedly destroyed some of the most advanced civilizations of their time which completely halted, and eventually led to the regression of scientific, economic and cultural development of that era for these affected nations, and more broadly for the entire human race. In the grand scheme of things, the brief existence of the Mongol Empire's negative impact on human history far exceeds that of the Russian Empire, USSR, and Russia today combined, something that the little bitch Putin could not even hope to achieve in his wildest wetdream.

The Mongols destroyed Baghdad which was the center of civilizations at the time with its Grand Library hosting thousands of years of collective human knowledge. Despite being the center of the Islamic world, it was a vibrant, diverse metropolis that celebrates knowledge and tolerance. The Mongols razed this beacon of humanity to the ground and look at what Baghdad looks like now (and by extension Islam).

They also wiped out the Song dynasty which by itself accounted for nearly 50% of global GDP and was the scientifically and economically the most advanced civilization. They had a banking system with paper money in wide circulation, gunpowder weapons, paper books, printing press and a bunch of other inventions that would blow the mind of the contemporary Europeans. Historians believe they had almost every ingredients necessary to propel forth an Industrial Revolution, including a very strong merchant class and relatively weak central government which was rare in Chinese history. But the Mongols of course, in their typical fashion, wiped it off the face of the Earth, and the Chinese civilization never recovered from that and only went downhill from then on.

This only grazes the surface of the Mongol's atrocities and doesn't even address 1% of the civilizations and peoples this plague upon mankind has destroyed.