Just like the A-bombs, The Rape of Nanking Saved Alot of Lives
The estimated 300,000 or so civilians killed in Nanking by occupying Japanese troops put an effective psychological impact on Chinese combatants who were fighting a losing war to become convinced of the futility of resistance and put down their arms. If it weren't for this massacre, not just 30 million, but 50, even 100 million Chinese would've been killed in the 2nd Sino-Japanese War.
Yet, both the Allied and Japanese sides think this event was a bad thing. Even Japanese nationalist perceive this atrocity as an un-necessary waste of life that they deny the high death toll, if not the whole event itself!