Mutual Assured Destruction works when you have sovereign stable governments with their fingers on the button. The more likely scenario is Iran supplying one its surrogates (i.e. Hezbollah) with a tactical nuclear weapon which is launched into Israel and not readily traceable to Iran. Obviously they dont care about civilians. Then what?
Donald Trump: Israeli settlement construction in West Bank should 'keep going' (Haaretz)
from the article: Let them have nukes