I dunno, the conspiracy theorists back in the 80's thought Reagan was insane and wanted to start a nuclear war so some Illuminati-types prepared for it could emerge from the ashes and repopulate the world in the name of Baby Jesus or whatever. That idea is what inspired alot of media from around the time like fall-out's 'enclave' and the nobles in Vampire Hunter D, ect.
If you believe in conspiracies then you could suppose the elite is orchestrating events to lead up to world wide depopulation.
Anyways, that's off topic. I kinda forgot China and Russia backs up NK because they kinda like having that buffer state there and don't want US bases built next to their borders. Maybe these countries are starting to become less tolerant of NK as they are doing more business and their interests are more aligned with the west since the cold war ended. If North Korea becomes too isolated and loses their backers they might get desperate and start a war if they feared an invasion. China could serve it's interests by intervening and occupying as well, they could replace North Korea with another entity to act as a buffer.
South Korea does not want 'reunification' because they don't want to deal with rehabilitating an economically defunct north and impoverished backwards refugees flooding their cities. So some other nation state would replace it, hopefully with better management.