There’s no defence against submarine based warheads; enough will get through.
It’s the reason that NATO isn’t going establish a no fly zone over Ukraine. They know it too.
The guy is 70 years old. An old, lonely, bitter man. His wife left him in 2013 and his kids are far away from him. All his Wealth is blocked, he will never be able to enjoy his life outside of his little empire. He has nothing to loose.
He could choose the bunker, but maybe he decides to bring the whole world with him. And if he decided already, nothing that we do, will make him happy. We are just deciding to die without even a fight.
The song that describes Putin right now:
"I am alone, this bitter man that I've become Hold my breath, laid to rest Drown in my tears, there's nothing left of me I am alone, the angry man upon his throne I can't pretend, the great descent Take my hand until the end You see me on the horizon I am the air you breathe I sold my soul to the seven seas Condemned and lost, pulled off course I will never leave, I'm always yours"
Any given day - Bitter Man
I can’t see what he has to gain from this war. There’s no upside for him.
All I can think is that he’s got something like Parkinson's, knows he’s on the way out and just wants throw his weight around one last time.
Honestly though your guess is as good as mine.
But if we still fear the atomic war, he can still threatens Europe with that.