Putin may be ill. If he has little time left he may want to go out like some sort of Peter the Great, in which case he could be extremely dangerous, and since he has started this failure of a military operation, he has painted himself into a corner where failure could end his career abruptly from the inside of Russia with great humiliation for him.
Stated nuclear policy of Russia includes any existential threat to the Russia(n government), and Putin has created that situation of potential failure in Ukraine.
Good points. Thanks.