If I may, I recommend 1162, the year that Temujin - a.k.a. Genghis Khan - was born.

Because during his life it has been estimated that under his command, the Mongols killed as many as 80 million people and wiped out entire civilizations from the Korean peninsula to the Nile, including world centers of learning such as Baghdad and Damascus and several of the great cities in northern China. His descendants ruled China, Russia, Manchuria, and eastern Europe...and in a region of South Asia, his descendants' rule did not end until the early 1900's. There were civilizations that were destroyed so utterly that they exist now only as rumor or supposition, with little or no directly written record of their existence.

It has been posited that the Islamic world, once the most educated people on the planet, have even now not yet fully recovered from the Mongols' genocide.

The Black Death killed more people, yes...but the Mongols didn't kill just people, but entire civilizations. That's why I point at 1162 as the worst year in human history.