If China wanted that buffer zone to remain intact, that interest would be best served by a well behaving North Korea, wouldn't it? I don't think the buffer zone accounts for China's behaviour.
I wouldn't put it simply down to just being a buffer zone, but I do think it is important for them.
China and North Korea has a shared history of politics and agendas for a long time. In the game of modern geo politics, having a well behaved NK may actually have the effect which they are looking for. A disruptive NK means that SK and the US are kept busy.
But also do not forget that China also has to protect China from NK. US plans may just destabilise the area, and I am quite sure some US Intelligence officials know this and are taking advantage of the situation.
If the North Korean government collapsed, I would assume that huge numbers of refugees would stream over into China, which is certainly not something they want to deal with.
China and North Korea has a shared history of politics and agendas for a long time. In the game of modern geo politics, having a well behaved NK may actually have the effect which they are looking for. A disruptive NK means that SK and the US are kept busy.
But also do not forget that China also has to protect China from NK. US plans may just destabilise the area, and I am quite sure some US Intelligence officials know this and are taking advantage of the situation.