I think people are missing the point a bit about the relationship between North Korea and other countries, most notably China. In the past, China has exerted massive control over North Korea and has been able to use them as a deniable asset to do various things for them. This might just be generally keeping people on their toes, but also other stuff. What about cyber attacks. North Korean attacks are often traced back to China. This and other information implies a good deal of cooperation. For China, this means North Korea can do things for them (such as cyber warfare) and if things get a bit hot, they can simply be cut loose or sacrificed. Not so if the PLA are doing it.
However, the above only works if China has positive control of what goes on. In the past (prior glorious leaders), this always seemed to be the case. However, with this latest change of North Korean leadership, Chinese control seems to be waning. The glorious leader seems to be doing things that are annoying China and even directly against their wishes. In other words, China is loosing positive control of North Korea. At this point, North Korea becomes a significant issue to China and if anyone is going to invade North Korea, China is best placed to do so. They could even sell it as a humanitarian act.
Comments about China being worried about a US influenced state on their borders (if say South Korea invaded north or whatever) are really wide of the mark. China knows nobody is going to try and invade them. Their forces are so numerous that nobody stands any chance of victory and would be dragged into a massive war of attrition. So, China isn't worried at all about who's on that border. Even if it was a unified Korea with the USA standing behind them, it wouldn't worry Beijing.
North Korea is a handy puppet for China, but when they become too hard to control, China will deal with it, probably through leadership change. Be in no doubt about that.