The Roman Empire engaged in genocide and slavery against many of their adversaries, yet Rome is still viewed as the peak of ancient civilization. The USSR imprisoned millions and caused the deaths of tens of millions more, yet leftists try focusing on the free healthcare and education. The United States engaged in ethnic cleansing of the native Americans, yet given enough time, such crimes have faded in saliency. Manifest Destiny was actually the inspiration for Hitler's ideas of lebensraum, invasion of Russia, and genocide of the Slavs. If Hitler had won he probably would have been considered more akin to Caesar or Napoleon. History is written by the victors and all that.