Well, this wasn't asked of me, but here is my answer.
Firstly, I think there is a distinction to be made between defensive support of an ally and faciitation of aggression against a civilian population. I would have supported a targeted, intelligence-led, ideally jointly coordinated and overseen operation, carried out within international law, against the terrorist organisation responsible for pretty much the worst terrorist atrocity since 9/11 (even knowing that people and places I care about would have been in harm's way). I also think international laws and limits should be applicable for all and that governments should not expect to be given weapons by countries who claim to uphold those standards without also being expected to adhere to those in the use of the resources made available. So, I don't think governments should say "fend for yourself", but I also don't think they should be giving a free pass when it comes to international law.
In your hypothetical situation, a country and its civilian population would be at risk of harm and potentially destruction. Regardless of whatever went before, the right thing to do would be to protect the innocent population at risk. Just as innocent Palestinians do not deserve to die because of the heinous actions of the people in power, nor do innocent Israelis deserve that fate because of whatever acts of the leadership in your scenario led to the international community stopping military assistance in the scenario.
I would point out that neighbouring Arab countries have been involved in trying to protect Israel. When Iran launched missiles and drones, countries helped their neighbour. It would be likely that a defensive coalition to protect the existence of Israel would also include other countries in the region and that even countries which might 'keep out of' actual direct interventions might contribute in other ways. Israel is not alone and will not be alone. Its government should still be expected to conduct itself in accordance with basic ethical and legal standards though.