I keep seeing that people think a test site collapse caused this, but if that was the case it isn't like they were going to suddenly be invaded. I think if anything, something internally was to cause this negotiation of peace.
Regardless of motivations, if this is a sincere move towards peace then it is a good thing for the world.
If Kim Jong-Un can pull things off right, he and his familiy will be able to live out their lives without having to fear being deposed and murdered, probably keeping enough wealth to live in luxury for generations, and maybe going down in history as a well-regarded peacemaking hero on top of it all.