Success and greatness of historical figures do not mean they were moral heroes although people would like you to believe otherwise. King Example XIV of the 15th century might have been a really good guy but he never did anything so nobody knows or cares about him. His half brother and uncle killed him and invaded and subjugated 6 neighbouring states but was less bad then everyone else at the time so he's great and successful instead of a villain. I don't think there are rules, but i think anyone who generally advances progress instead of reversing it is considered great. Genghis Khan as far as i know didn't so he isn't. Julius Caesar did so he is.