Mirror Universe-Terran Empire-How?

[xenexian 2]

Even though I'm an old Trekker, I'm always still fascinated by the mirror universe.  I've tried to research how the humans came to be so bad and I'm not coming up with much in regards to background.  One of the more plausible explanations I've read was that the Terran empire was the result of the Romulan war.  After losing the initial war (Capt. Archer's era) the Earth was occupied by the Romulans for close to one hundred earth years only to be finally overthrown by the human capacity for not tolerating captivity.  As a result, humans began a program of aggression that the Alpha Quadrant had never seen before.  Never again would humans be subjugated to occupation and conquest, from now on, they would be the conquerors and hold human kind above all.  Were the humans simply responding to their environment or was the mirror universe humans an anomaly, considering our nurturing history, and was a result of a hostile universe and a breaking of human ideals with how we treat those that are different?  Were the humans in the mirror universe a product of what they endured or was the capacity for aggression simply suppressed because of human society and then released when they became conquered?  The idea of an aggressive earth fascinates me because of our capacity for both aggression and peace.  What drove us to relinquish what peaceful teachings we had over millenniums of religious history?  What exactly happened to Earth during the occupation?  Enquiring minds want to know.  If anyone has any other insight  as to how the humans became the scourge of the universe it would be greatly appreciated if they would share.  If not, I have a great  book idea formulating.