The allies' strategic bombing campaign was intended to destroy industry and infrastructure and was not aimed at any political group in particular, whereas Israel can and does target precise buildings associated with Hamas (see the "Lavender" program that provides bombing targets).
> That's not even what Israel is doing now
There's no real way to know since Israel does not allow journalists in Gaza, but the international court of justice found there was sufficient possibility that an investigation should be carried out. Are you so much better informed than them that you can be sure?
The ICJ is empathetically captured by the suffering of innocent Gazans. Nevertheless, the suffering of innocent Gazans is not Israel's problem as Israel is trying to prosecute a war against Hamas, as long as Hamas is intentionally hiding behind and under civilians and civilian infrastructure, and as long as Israel isn't intentionally bombing civilians and civilian infrastructure without a justifiable military target behind or underneath them. The idea that the ICJ is going to second-guess every military risk-justification analysis of "we might hit this many civilians, or a hospital, or a mosque, to get a few known Hamas militants or a weapons cache" is absurd. I don't know if Israel is getting it right. I do know that the ICJ can't possibly get it right, because they will disbelieve everything Israel says, and believe most Hamas-influenced Gazan representations.
Israel could have carpet bombed Gaza without warning anyone to move, without doing roof knocking etc. But they did warn, and they got roasted by the international community for being so inhumane as to herd people out of their homes. Israel just can't win in the court of Palestine-sympathetic international opinion.
You asked whether Israel will kill millions to avenge thousands.
It doesn't seem as much like vengeance, as a refusal to continue to sit and do nothing while Hamas uses, or tries to use Israel for Qassam rocket target practice. (Iron dome and Iron Beam aren't perfect, and I'd expect low-flying IED drones if this conflict continues.)
The Gaza Health Ministry (which is basically Hamas propaganda) estimates 50k dead. How do you see the death toll getting into the millions, unless you think Israel is going to abandon its current tactics and start massive bombing campaigns in areas without warning people first, or unless the war goes on for decades?
You're comparing the Allies' actions after WWII concluded with Israel's actions in the midst of conflict.
Are you forgetting the Dresden firebombings?
Are you really suggesting that Israel will continue to "kill them all" if Hamas surrenders? That's not even what Israel is doing now, although they have the military capability to do so if they wished.