I agree with what you said about proportionate responses, but you are off when you mention international law. It applies to more than just relations between states. You’re interpreting the word “international” as meaning “relations among nations” when a better interpretation is “relations among and within nations.” International humanitarian law applies to armed conflict, whether of international or non-international character
You might be mixing things. Here is a link from the International Red Cross explaining the difference:
See this too:
The term (public) international law strictly applies to relations between states or international organisations.
international humanitarian law applies to the parties that are signatories to the relevant conventions, it doesn't apply automatically everywhere.
even crimes against humanity are not such because of international law, they are crimes because certain contracting parties decided that they are crimes. Genocide and slavery are prohibited not because of international law, they are prohibited because the members of the UN decided to prohibit them and some gave themselves universal jurisdiction over them.
I understand your point and there is a "creeping" of "international law" into things like protection of the environment and many other aspects of internal law. But it is wrong to say that "international law" prohibits something. A treaty prohibits something once it is ratified and becomes part of national law.
But Gaza is not a state. Israel is bound to follow the treaties it has adhered to regarding international humanitarian law (which apply even to police procedures - e.g. prohibition of torture), but not to the rules that apply to fighting regular armies. A soldier that shoots another soldier in a legitimate war cannot be tried for murder.
A terrorist, however, can be tried for murder and his accomplices too. Gazans that attacked civilians are terrorists. If you claim no, they are soldiers in a war, then they are war criminals. If you claim that those who attacked Israeli soldiers were soldiers in a war against Israel, even so, they committed war crimes by torture etc. We know what they did.
Take a look at perfidy (prohibited as a war crime):
Hamas as an organisation advocates for genocide and so puts itself outside of the protection given to soldiers and makes every single one of them a war criminal or a terrorist.