Exactly this, you cannot let a nuke go off without a reaction. Using a nuke against a non-nuclear state, especially as the aggressor, can only be met with a similar response. The global impact of that level of escalation will also be severe, as countries naturally allied to Russia will then consider themselves targets of tactical nuclear weapons should they face the US and her allies. So it's fundamentally a scenario that nobody wants to see and Russia will find itself even further isolated. Beyond that it's MAD and to be honest we might as well just watch Kevin Costner's The Postman or Mad Max to understand the longterm consequences.
I do, however, understand why Russian rhetoric is ramping up internally. The propaganda machine can only sell Russia as a militaristic superpower, something that is pretty easy with 6k nukes. Ultimately, it's all they have. I personally don't think that nukes of any description will be used and that fundamentally the worst case scenario for Russia is that they do a full scorched-earth retreat and claim they've cleansed Eastern Ukraine of whatever made up shit they can think of. No matter what happens, they've already lost any future war with NATO due to their tactical blunders and the sheer determination of Western countries to starve Russia of the capability of modernising any further in the future.
