I think that mr. Putin wouldn't be too concerned if N.Korea launches a missile against USA

But as far as N.Korea and her nuclear threats I think its all about forcing some negotiations about sanctions and food aid. The country is very poor and with technological backwardness they have all sorts of problems. Also to keep people of N.Korea under regime Kim Jong Un must always find some imaginary threats. That way people are willing to sacrifice themselves for the nation and their leader.