Thinking about a conflict between nations or large groups of people in terms of common law is not the right approach. Before going further I do want to say that

Thinking about a conflict between nations or large groups of people in terms of common law is not the right approach. Before going further I do want to say that I do not condone torture, prisoners of war and/or detained civilians should not be subjected to violence after they have surrendered or been captured. That being said, the only way to think about this is in terms of a war. Israel has officially declared war on Hamas which is the ruling body of the Palestinian territories. A war does not end when the soldiers involved in the first battle are killed, it does not have to only be defensive and it does not have to be proportionate. Other nations may attempt to influence the conduct of nations who are at war but the only recourse beyond political and economic pressure is military action. Israel has no obligation to stop the war until they have achieved their goal which is understandably the complete elimination of Hamas and an end to any further attacks on Israel. This is both a reasonable and an achievable goal given the territory involved and its proximity to Israel. They are not fighting a war halfway around the globe for reasons that may hypothetically affect them at some point in the future. They are fighting on the very borders of their country after just the latest and most horrific attack out of a long history of sustained violence against them. So yes I do think your proposition is unreasonable, I know that if my country was in the same situation that they would respond in the same way and I suspect every last country in the world able to defend itself would do the same.