China invades from the north and west
unless your belief is a couple of border guards with sidearms will stealthily remove the fat wun, do you believe the Norks won't somehow notice the build up of forces on their border? You think the Norks haven't noticed the degrading relationship with the Chinese? The Chinese have been on the receiving end of NK propaganda, so they are willing to bite the hand that feeds them.
Knowing that your idea would require a significant build up of forces to assure success, which would attract attention, do you think the fat wun wouldn't unleash his nuclear fury on the Chinese if he believed they were intent on unseating him? What's to stop him?