The problem is Russia can't defeat NATO without nuclear weapons.

What's stopping Russia from introducing nuclear weapons in that case?

Probably MAD, as in the Cold War times.

There's still the hope that nobody, not even an imperialist Putin, would be willing to risk the total destruction of his own country if not of human civilization over some territorial ambitions.

What if he is actually mad? Maybe terminally ill and not minding the scorched earth he'd leave behind?

Luckily, this is Russia. There is always someone willing to stab the tzsar in the back when the time comes.

"What if he is actually mad? Maybe terminally ill and not minding the scorched earth he'd leave behind?"

So the solution of the OP is to be madder than him and start nuclear war first?

I wish there were someone to replace the mad tzar, its long overdue

I would say, so far Putin is proceeding rationally. He prepared his economy for this over many years, made strategic agreements with opponents of the West, and slowly built up a force which will now take a neighboring country using classic military techniques and massive imbalance of power.

Maybe the word that better describes it is 'systemically' or 'meticulously'. When I am in a mood to get drunk, I will prepare by buying the alcohol, invite friends, prepare the food, and then get myself piss drunk. Not sure if the world 'rational' fits ill conceived plans that are just well executed.

Even Putin wants to live. And he probably doesn't want to go down in history as another Hitler - even to his own people (the remaining ones, that is).