The mentality of the Japanese military during WW2 was a bit over the top. In the case of Nanjing, the much larger Chinese army defending the city just left.
They had communication issues, and everyone ran, leaving the city undefended. The Japanese army outside the city thought the Chinese soldiers were cowards, and then when they encountered the civilians, things really went downhill. The locals of Nanjing were very obedient and docile. The Japanese had no respect for people who gave up without a fight.
Apparently, they were the Klingons before the Klingons. The end results were they got away with everything, and they committed war crimes on a scale not seen since the Mongol Empire punished cities. They murdered babies, killed females of all ages after they were sexually assaulted, conducted mass beheadings that created rivers of blood, etc.
They did things that people can't even imagine. Every soldier that participated thought they were some sort of samurai. They are many pictures online that are NSFW that you might want to look at. They are also documented stories from survivors and witnesses such as Americans and other embassys that were in the safe zone during the massacre.
