If cornered they won’t back down; along with bio weapons they have a variety of thermobaric bombs, and some “unknown unknowns”.
Russia has not deployed their full arsenal yet, wanting to minimize civilian casualties and infrastructure damage. That comes next.
Ukraine is cornered, Russia is being left alone and untouched.
Your theory they want to achieve international pariah status by going ‘total war’ WMDs for an ordinary foreign invasion seems unlikely.