Adding on to the other answers, Japanese soldiers were expecting an easy victory in China, and when Japan started taking far more casualties than expected almost immediately, the Japanese reacted brutally: partly in the hopes of causing terrified acquiesce, and partly because the Japanese soldiers were just that furious.

It might seem odd, but Japanese training actually emphasized brutality as a means to make up logistical shortfalls, with beatings and bizarre punishments common in training as a result. Naturally, civilians under them were treated even worse, and this wasn't helped by Japanese propaganda lying about how "grateful" the locals were to Japan, which made the subsequent local resistance also feel unjust at best, leading to worse reprisals.