There is a pretty realistic nightmare scenario where the US withdraws support from Israel, its massively more powerful neighbors (notably Iran) start attacking it and giving it an absolute shellacking including tons of dead civilians in the streets and its territory crumbling day by day to an invasion, and then it decides to "defend" itself by starting to throw nukes around, because it's game over otherwise so why not.

I don't think even Netanyahu would attack someone with nuclear weapons simply as part of their ordinary murder geopolitics, but for an existential threat I think it's completely realistic, and I also think it's completely realistic that Israel without Western support would face an existential threat.