This question suggests some confusion on your part in terms of the difference between history and ethics, as topics of study.
The historical evidence shows that Ghengis Khan was responsible for a vast number of deaths - horrible deaths, and these are well documented. That's what history can talk about - the facts and the context.
As for the moral question of whether or not mass murder is morally acceptable, well, I invite you to do your own research but the consensus- and my own opinion- is that no, it is not.