Ghengis Khan was one of the greatest of mankind's benefactors.
By conqurering China, he shook that advanced but decadent culture out of its stupor, and began a new Golden Age of discovery.
By taking Chinese advisors with him on his conquests, he spread Chinese ideas all through the Middle East, shaking up the decaying Islamic states.
His deeds allowed Marco Polo the chance to bring Chinese ideas to the West - something the insular Chinese would not have allowed without a Mongol ruler.

You see? You can justify any barbarism by pointing to the incidental good it does. Remember that, when you hear the arguments about Gitmo and Iraq.