First off, chemical weapons are among the most appalling weapons that mankind has come up with. The elimination of chemical weapons should be a priority for the international community, along with nuclear weapons, landmines, cluster bombs and other appalling weapons. What I am trying to get at here is if you think that some weapons are so "beyond the pale" that they should never be used in any circumstance.
This is a serious question and I don't want to get derailed into the rights and wrongs of Israel vs Palestine, Lebanon, Syria, or any of the other conflicts over the years.
According to US intelligence, Israel has an arsenal of chemical weapons in addition to what is at least a small nuclear arsenal. Israel, like Syria, is not a signatory to the treaty on chemical weapons use.
Israel is surrounded by countries with large populations, and many believe that "the Muslims" are implacably hostile to the idea of Israel and is really just waiting for the day that they can push Israel into the sea and do horrible things. Just for comparison, Israel has a population of 8 million and Iran has a population of 76 million. If you look at the Iran/Iraq war, both countries had the ability to put very large number of troops under arms which led to casualties in the millions. (The response of the international community to Iraq's use of chemical weapons in this war is instructive.)
So take your pick of a hostile neighbor for Israel. The historical tactics for the use of relatively untrained soldiers against a technically superior enemy is to overwhelm the enemy by numbers. This tactic has been used as long as men have fought and while it leads to a lot of casualties, it can be very effective. This is why poison gas was invented as a military tactic.
Assume that Israel has a hostile neighbor committed to destroying it at all costs, with a fanatical population willing to die for their beliefs. Assume again that despite the odds (or maybe not; China overwhelmed technically superior US forces in Korea by intelligent use of numerical superiority) the implacable enemy is achieving success and are closing in on the Tel Aviv. The consequences would be horrific.
So you are a commander in Tel Aviv facing, say, 6 million enraged enemy (backed by better trained troops) and unaccountably your air force has failed you. (You have discovered that relying on high-tech tools makes you vulnerable to hacking, and Israel is not the only country with talented programmers.)
You have a supply of sarin available to use as a last resort.
Do you let Tel Aviv be destroyed because chemical weapons are immoral? Do you use nuclear weapons because that is somehow better? Is anything fair when facing an existential threat? Do you adhere to every jot and tiddle of International law? (Shush, I know about 242.) What is your choice?
If not, how is your position any different than Assad's? From his point of view, Syria is facing an existential threat. It has been well documented that at least some of the rebels are committing atrocities, so from his point of view this is not an academic question.
I really am interested in your views, because I'm pretty sure that most people would use anything at their disposal to save their own city. I'd be willing to bet that if this scenario happened, a large number of people here would support whatever Israel did.
Now, for the interesting question. Suppose that the Israeli commander did decide to use a bit of Sarin to save Tel Aviv. What should the US do in response?