The Romans also did the same when they conquered land. Just imagine the countless souls lost their lives at the colosium just to provide entertainment for these sadistic people. I have a huge respect for Atila the Hun and the Germanic Barbaric tribes who crippled the Roman empire.
The irony is that western historians consider the mongols as brutal inhuman rulers but praise the Romans. At least Genghis Khan let the people of the conquered land live freely and gave the freedom to practice their religion and traditions. He only slaughtered those who opposed his rule.