I think we need to view it from Putin's angle. What will keep him in office (because if he loses office, he might very well lose his life, certainly his freedom)? 
One possibility is that by dropping a small nuke, that hopefully will only be met with a conventional weapons response, Russia will then be isolated from the world, and Putin imagines he can survive in power, 'forever' raging against the West, much like North Korea, or Iran to some extent.
Other options are that Russians themselves remove Putin before he can drop a bomb, or that somehow he negotiates a withdrawal, but I don't see other Russians allowing that, but maybe there's more resistance within Russia than I realise. 
Scary times indeed!