@Ekfreet Putting it harshly, rape is pretty much one-sided sex where the other suffered. Is it cruel? Yeah, my country is pretty much fucked when Japan invades despite all their promises of being the savior, but there's likely to be reasoning there. I use quote-on-quote there, because "greater good" here means being effective. The German in WW1 uses Mustard Gas, and even if we ignore how disgusting its effect is and how it can even affect unrelated civilians, but it'd be stupid to ignore the reason why they want to use it in the first place and how many German soldiers were saved by it.
For the Japanese in WW2 all those crimes are useful to win the war and become their own major power instead of being toyed around by the factions in Europe and America. If they fail, the time where Japan needs to become isolated again might be necessary if they want to stay independent, and that likely won't last for 50 years.
Also it's a bit annoying to point out (for both you and me), but while it is a very cruel and meaningless thing to do for them to kill the baby, there's nothing mentioning about it being a competition and stuffs (even if there is, I don't think any of them mention any source but I might be wrong). A bit unrelated, but the other alternative for the baby that's possible is either dead by starvation or a survivor to pick him up (which probably just led to the same end).
And like I mention before Jack the Ripper's crimes have none of that. There's no clear motive that could make it count as "effective" for him.