When I was in university, in Tokyo, there was another student (a white American) who said that he had read some diaries from Japanese soldiers who were there during the Nanking Massacre.
I make no claim to the accuracy of this information and have no cites. I am merely relaying something which is maybe true that an interested party could try investigating if he has an interest.
So the way he explained it (if my memory is correct) was that officers of the Chinese army had a standing order to shoot retreating soldiers, and were armed well enough to be able to carry this out fairly effectively.
When the Japanese army came, the Chinese soldiers in town realized that they were going to be massacred if they stayed and so they started to murder the regular citizens to steal their clothes and possessions, so that they could flee, under the guise of being a non-soldier.
As said, I don’t know that this is true. And if true, I don’t know how large a component of the massacre it was. The Japanese were certainly rather horrific in general of their treatment of the people they encountered, so it’s plausible that even if the Chinese army did commit a large number of the deaths that if they hadn’t done so, the Japanese would just have done the same damage.
On Okinawa, a few villages committed mass suicide, rather than be taken prisoner by the Americans, because they thought that their treatment as prisoners of war would be the same as the Japanese treated their POWs. As said, they were pretty evil.
But then again, I’ve also heard that half the Japanese military deaths were down to starvation. (I believe they said this in a CrashCourse video, but I haven’t been able to find a confirmation of this anywhere.) If they were dependent on stealing food from every town they took over, it would make sense that they’d kill everyone they encountered, rather than suffering them to starve. Likewise, they wouldn’t care much about POWs, since they’d be second priority to feed.
It’s also worth noting that the Chinese government, after the war, proceeded to cause the deaths of ~40 million of its own people. It’s not like we’re really comparing a good country to a bad country. The people of China were doomed either way. Even if we accept that all deaths and rapes that occurred in Nanking were solely down to the Japanese, the Chinese has far more to apologize for, to the Chinese people, than the Japanese do. And of the two, Japan has come far closer to actually doing so.
Ultimately, there’s no rule that one side has to be the good guy and one side the bad guy. In that time and place, there were definitely only bad guys. I’m pretty willing to believe that Nanking could have been a shared atrocity (though I’d lean towards the Japanese being dominant, despite anything that the Chinese soldiers may have done).