I have this feeling if Kim Jong-un suddenly is off the scene permanently as leader, the North Korea government collapses in no time flat and within one year, the Korean peninsula is unified again--this time under South Korean control. And it could become a tremendous economic opportunity for the South Koreans as they now have a place to use their mighty industrial output to upgrade the infrastructure of the former North Korea.
They gave up the idea of full-scale conventional war. That is why they are obsessed with strategic weapons. With nukes delivered by missiles, they think they can scare away U.S. because those missiles can hit U.S. bases in Japan and Guam, and U.S. mainland. Then they will intimidate S. Korea. They create military provocation and threaten to nuke S. Korea if S. Korea tries to retaliate. They attempt to offset their deficit in conventional arsenal with strategic weapons. Their goal is to intimidate S. Korea enough to become its satellite. Probably an implausible goal, but none the less the one they have been pursing for a long time.
Of course, they also see it as the only way to protect their regime.