There wasn't really a general Japanese opinion of the events of Nanking. While there were news reports "celebrating" the so-called "hundred head race," where two officers had a contest in how many heads of people they could chop off, there were also examples of regret, especially among the higher staff. The general of the forces in Nanjing, Mitsui, expressed regret for his troops' actions and openly accepted responsibility for the Nanjing massacre at the War Crimes tribunal.

You also had a few soldiers after the war that publicly admitted their guilt, describing several of the crimes they had committed (including an incident where an officer placed a man in a mailbag and tossed a hand grenade in it).

And then you had people like Kenji Ishiwara, who was definitely one of the most outspoken of the non-expansionist faction of the army. Despite being virtually single-handedly responsible for invading Manchuria, he proceeded to become a steadfast anti-expansionist, going so far as to personally attempt to negotiate with Chiang Kai-Shek in 1937. He was overruled by the expansionist faction, including Hideki Tojo. Ishiwara hated Tojo, calling him "Lance Corporal Tojo," criticizing his actions, refusing to send reinforcements to Shanghai, and finally advocating for Tojo to be arrested and executed (at which point he was forced to retire).

It really depended who and when you asked someone.