Russian senator Konstantin Kosachev warned that after the referendums, "protecting people in this region will not be our right, but our duty. An attack on people and territories will be an attack on Russia. With all the consequences." Russian Foreign Minister Sergey Lavrov did not rule out the use of nuclear weapons to defend annexed Ukrainian territories.
Source: https://en.wikipedia.org/wiki/2022_annexation_referendums_in...
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.