Apparently, his motives were like gods' by definition.

from: http://www.fsmitha.com/h3/h11mon.htm

"Like others, Genghis Khan's subjects saw themselves at the center of the universe, the greatest of people and favored by the gods. They justified Genghis Khan's success in warfare by claiming that he was the rightful master not only over the "peoples of the felt tent" but the entire world."