In war there usually is no good or evil side, just two evil sides believing they are the good side. The allies even had a nation on their side that committed an atrocity roughly equivalent to the holocaust in the case of the Soviet Union with Stalin and holomodor:
https://en.wikipedia.org/wiki/Holodomor
The victor makes themselves out to be the good guy and the opposing side as the bad guy, but they're just two sides of the same coin. The U.S. has been pretty horrendous when it comes to wars and their atrocities not even including covert indirect atrocities done by the CIA (nuked Japan twice as one example of a direct atrocity out of many), but it's human nature for the strongest nation or power to commit egregious crimes when they have the power to do so.
I suppose the only difference between nations and powers is the extent of the severity of their crimes, but no empire has their hands clean because to get power you must first get your hands dirty. Then once they get power, it corrupts the nation or empire. History has proven time and time again for this to be true for nearly every country and empire to have existed. So all you really have in the end is that X nation or empire is the lesser of the two evils compared to Y nation or empire.
The public just goes along with what their nation or empire does out of either ignorance of what their country has done (U.S. is a great example of that) or from propaganda that has convinced them that their country is fair and just. To think Germany in WWII is exempt from this rule because of the allies' account of WWII history is laughable. It all stems down to human nature being corrupted by power because society is just a macro extension of human nature.