This is one of the things that I can't really find a satisfying answer by myself...
Through out the Ages there have been numerous large and diverse Empires that formed, and then disintegrated...the question is: was their reason that an Empire was built and the conquests were done for a greater good (peace and unity) can be justified...?
I mean, the Romans slayed thousands of Carthaginians, Gauls, and Greeks so that "peace" can be built on their former lands...the same case goes to the Persians, Macedonians, Arabs, Franks, Mongols, Ottomans, Spanish, French, British, etc...
So was the formation of an Empire can be seen as the right way to achieve a lasting peace...?