Of course the peace with north korea went bust. They had Jong Un killed because he wasn't barking whenever china yanked on his leash anymore, and north korea is a useful wall that denies an easy land route to china's vulnerable interior.
A unified korea would rapidly westernize and you'd have berlin wall 2.0 along the china/korea border as droves of chinese would try to jump the fence to freedom.
Those are all direct threats to the communist's internal narative that cannot be tolerated.
https://www.nbcnews.com/news/north-korea/beautiful-letters-dark-nightmare-how-trump-s-north-korea-gamble-n1230866