(1) They were ruthless. They weren't afraid of using whatever means was necessary to conquer other civilizations & cultures.
(2) They were bloodthirsty. They used force to get what they wanted. They weren't above decimating entire ethnic groups, civilizations, & cultures. Attila wiped out the Burgundians. Genghis Khan exterminated the Tatars & Hitler all but exterminated European Jews.
(3) They wanted world domination. Genghis Khan was the only one whose empire lasted for centuries. Attila & Hitler had short-lived empires.
(4) They were egotistical. They believed that they were destined to do great things.
(5) They dealt harshly w/people who opposed &/or were deemed threats to them.
(6) They instituted their version of a new order.
(7) People dreaded, feared & hated them.
(8) They left less than a positive historical legacy.
(9) They believed that their people were superior & others were inferior.