N Korea is on the verge of an economic and political crisis.
They can't feed their own people and the Glorious Leader is crazy or dying, and probably both. They're the international equivalent of a disgruntled postal employee, and they're desperate to do anything to distract their own people from their imminent collapse. It's a national murder-suicide pact.
I don't see them as a military threat. Their bomb is a probably a dud, and their conventional force is formidable, but it has no logistic sinew to drive it. A country that can't manage a peacetime economy and hasn't fought a war in half a century is going to have a hard time delivering bullets, beans, and bandages to the front lines. The ROK will mop 'em up.