It would also be the end of NATO and probably the rest of the world. That's why it's called mutually assured destruction. But it seems like recently policy makers are forgetting that
They know they can get a few off if they sacrafice a few Russian cities. Which Putin does not care about. He'll watch them be vaporized if he thinks he can land a death blow on any major western powers.
The criteria given by the other poster for Putin to use nukes is currently met. So using their logic, he should launch them now. I'm asking why he doesn't.