Morality of War and International Law

Have a good week!
1- In terms of international law of war, a distinction must be made between harming civilians and harming soldiers (although a civilian loses his protection when participating in combat). I would like to argue that when fighting a culture that by its very nature does not accept this morality and educates by example that every Jew is a devil, then they should not be treated with the caution required by law (of course, morality always requires caution in killing, but the law is somewhat positive and not based on natural morality, and therefore morality is judged on a case-by-case basis). Therefore, even though it is not the fault of all citizens in the education they received, are we obligated to act in this way?
Please note that I am not making a legal claim regarding the protection that citizens who are used by a terrorist organization are entitled to, but rather that the law itself does not apply to them (but only ordinary morality)?
2- I would also like to ask, how much collateral damage is permitted by the moral and Torah rule of "If someone comes to kill you, kill him early"? If, for example, we know that a terrorist is about to bomb an entire city with a nuclear weapon and there is no time to find him, is it permissible to immediately bomb an entire country of innocent people when this is the only way to prevent this and neutralize the bombing?
In other words, is it considered that the defenders are killing the others and the only question is whether this is justified, or is the responsibility for their deaths the responsibility of the terrorist while the defense is not considered an act of murder?
3- What is the Rabbi's opinion on the common leftist claim that if more people died in Gaza, then this proves that we are immoral? That is, justice is determined by the amount of suffering.
Thank you very much!