The root cause of all the troubles at Nanking battle is the fact that Chinese leaders escaped irresponsibly without giving the remaining soldiers the proper order ignoring Japanese request to surrender.
On December 7 1937, Chiang kai-shek and Soong Mei-ling escaped from Nanking, leaving the responsibility to Tang Shengzhi.
December 12 1937, on evening, Tang escaped from Nanking, which led a big panic among Chinese soldiers and people. To be worse, Chinese source says he forgot to cancel the order to his troops to shoot anyone escaping. This brought a catastrophic disaster Chinese soldiers killed each other involving civilians at the gate and river (See Yijiang Gate massacre).