You think Putin is willing to commit suicide and see Russia disappear into nuclear dust?
Why do you see Russia as suddenly insane and wanting to go out in a blaze of glory, rather than restrain their aggression?
Wait. You think Obama would push the button? Hell effing no he won't. And Putin knows it. So the whole "safety because of the fear of mutually assured destruction" thing is off the table.
Putin knows a few tactical nukes here and there will go unanswered. Obama has assured Putin's advances just by being the lightweight he is.