Had a book assigned for history class that totally and forever changed my understanding of Genghis Khan. Genghis Khan and the Making of the Modern World I think it's called.

Dude and his empire basically singlehandedly spread the written word, religious freedom, and lots of other ideas we generally consider good today. Not saying he didn't kill a lot, because he did, but his historical impact is much more complex than "barbarian invader kills a bunch of people"