If Russia uses nuclear weapons against Ukraine, NATO will deliver a crippling response, as Jake Sullivan has said. There is no realistic scenario where Russia wins or holds on to its gains.
Putin isn't even the worst guy in the regime. The least bloody outcome would be a Russian revolution but I think that's asking for a lot of luck and hope. For a world that's ignored the risk of having nuclear weapons, we may now unwittingly be stuck in a trap in which using them is unavoidable.
https://human-rights.cmc.edu/2022/04/14/russias-genocide-han...
Countries actively avoid proliferating nuclear power because of the existing powers. Letting them be used against you is a strong signal and will weaken trust. It will become even more of an arms race.