1. This is warfare, not civil law. Hamas, which is the sovereign in Gaza (there has been no Israeli military presence there for years), commited an act of war in which hundreds were killed, raped, and kidnapped. Israel, as a state, has the moral duty to fight back and make sure this never happens again. Otherwise, it breaks the most basic contract between citizen and state ("I give up on violence and in turn you protect me from violence").
2. The aggressors (both in the field and leadership) are still largely out there, and holding kidnapped civilians and soldiers. They don't get a pass just because the "aggression is over" (whatever that means, rockets are still being fired indiscriminately at Israeli cities and towns, which is a war crime by the way...)
3. The responsibility for the safety of the Gazan civilians is that of Hamas since, again, they are the sovereign in Gaza. We shouldn't absolve them of responsibility for picking a fight with a better armed opponent. The defense does not have to be proportionate since that would mean Israel cannot use it's Tanks, warplanes, etc. To imply that Israel should fight with it's hands tied behind its back is ridiculous since war is not fair. There are some international laws intended to reduce the suffering of those that are uninvolved. For example, what does have to be proportionate is the harm caused to civilians when attacking a military target on a case by case basis, to the best of your knowledge (note that, according to international law, the target can in fact be a hospital/school/etc if it's used for military purposes).
Finally, an honest question, what would you do? How would you respond in this situation?