It's possible that NK has a legitimate fear of attack by the USA, who have attacked more nations since WW2 than any other country.  The list includes the likes of Grenada and Panama, along with Iraq, Afghanistan and Vietnam.  I'm sure there are others that don't come immediately to my mind.  

NK can point to those countries, all of which did not have the ability to strike the US as evidence that it needs ICBM capability to defend itself.  Perhaps the solution is a peace treaty to end the armistice and give NK the assurance it seems to need that it isn't going to be invaded like many other countries have been by the USA over the years.