I was watching something that mentioned the 2011 Tohoku earthquake, so I looked it up on Wikipedia. That led me to the 1923 Kanto Earthquake. In the opening blurb I see "Kanto Massacre". Which leads me to a page about the fact that, in the weeks following the earthquake, the Japanese just started slaughtering Koreans that lived in the Kanto region. Under the See Also, there's the "Gando Massacre", which was more of the same. Then there's WWII, and while I may not know too much about the things Japan did, there is definitely a very long list of war crimes and general crimes against humanity.
The only thing I know of separating that morally black regime that treated other humans as things to be slaughtered or experimented on and the country that many westerners associate with anime, video games, culture, and generally pleasant things is the occupation of the country by America after WWII. What exactly happened in those years that led to what I can only describe as such a drastic transformation in an entire nation?
This is a fascinating question from a sociological/psychological perspective. I get the impression there was a similar softening in Germany after the war as well. Could it be a guilt based, or a “please don’t hurt us anymore” sort of overcompensation? Maybe there’s something about surrender, defeat, and occupation that can change a country on a cultural level. I’ve wondered why some nations break and accept defeat and others keep fighting through occupation.