Putin's primary objective, from everything I've seen and read, is to be able to reassemble at least some vestige of the former USSR with himself at its head.

At this point, I'd say it seems to be an open question whether or how much it matters to him how much of that land is actually habitable.

It also seems like a) he views Ukraine as a somewhat rebellious province—daring to defy him, its rightful ruler—and b) he views Ukrainians as Russians, and furthermore believes that any Ukrainian who does not subscribe to this view is either traitorous or mentally ill...so I could see some fairly strong arguments for him deciding that they "deserve" to be nuked.

Honestly, in my view, the strongest argument for the position that Putin won't use nukes is simply that he hasn't done so already, despite many fairly significant setbacks in his attempted annexation of Ukraine.