Thanks heaps Matildemp and Itskt :)

Hello! I just finished an IB course (high school) focusing on Imperial China (Qing dynasty) and Mao until 1976, I hope I can answer your question before my next exam, so I'll have to be brief. Sorry!

Okay, so the Rape of Nanking (or Nanjing) is commonly seen as the most extreme case of the Japanese "three alls" policy in China, and this was done mainly because, as /u/itskt said, the Japanese wanted to establish dominance over the inferior Chinese.

What itskt hasn't mentioned, however, is the extent to which the Japanese felt undermined by the Chinese. During the Sino-Japanese war (1894-95), as well as way earlier during the Tokugawa era in Japan (1600-1868), China was seen as the "Middle Kingdom", and had Korea (close to Japan) as a tribute state. After centuries of this tradition, Japan evolved to perceive China as a sort of bully, and saw them as culturally inferior.

Preceding the S-J war, however, Japan was able to modernise quickly and effectively through the Meiji Restoration, and ended up inflicting huge losses on both the Chinese military (in the Battle of the Yalu River, which ended the conflict, the Japanese absolutely destroyed 8 out of China's 10 warships), and the Chinese people by damaging their self-respect and dignity, since it had been the first time since the 17th century that China had been directly threatened and defeated by an Asian nation.

This helped boost Japanese self-esteem, as it had been modernising in order to match the West and gain international recognition from it as its equal - something which it failed to do due to the Triple Intervention following the Sino-Japanese war. The Triple Intervention was what ultimately consolidated Japanese hatred for China, as Russia, Germany, and Holland basically bullied Japan into handing back the lands it had taken from China following the S-J war. This underscored Japanese isolation, and only fuelled its hatred for China, as it seemed that China was more important to the West than Japan was, despite Japan's military capabilities.

Japan acted so brutally during Chinese occupation because it wanted to both "repay" China for the humiliation it suffered following the Triple Intervention, and because it needed to establish itself as the superior nation in Asia. Also because if you break a people's will, you instantly get easier access to their resources, which China was chock-full of.

TLDR; The long-term cause was essentially that Japan saw China as a bully, and the shorter-term cause was the Triple Intervention.

The Japanese considered themselves (at the time) the superior Asian nation. They mistreated not only the Chinese, but many other Asian nations' citizens as well. The use of comfort women during WWII by the Japanese military is a great example of this.