Board Thread:Warhammer 40k General Discussion/@comment-24037808-20140924094901/@comment-6078851-20141001133946

the emperor may not have started as a warmonger but he most certainly became one: he united earth with the savage barbarity of his thunder warriors, in war. he then attempted to unite the lost worlds of humanity in the great crusade, the single largest military campaign the galaxy has ever known. the highest rank the emperor bestowed was that of warmaster. he did not allow empires the choice to live alongside with his own empire, it was either submit willingly, submit after i've kicked your ass, or be destroyed outright - the only exception to this was the treaty with mars since he didn't have the military might to conquer them, and that treaty then gave him the might for his war machine to plow across the galaxy.

granted there are species so hostile that all-out war is the only possible response, but the emperor unleashed all-out war on human empires that had survived, and flourished, on their own just fine and simply wanted to stay their own empire. even empire's that would've allied with the imperium wasn't good enough for emps, he wanted to have the lot under his domain and war was how he went about it, unremitting, all-out, war.