I think this way because China is not a good nation. They do things with thier people that NK is doing at times. We do not always see it because of the very tight control that China has on its media as does NK. That is why I do believe that China and NK probably have the VX nerve agent and are using it on thier own citizens. Why do you think they were able to release just enough to kill one person and not harm anyone else around him. This shows a more sophisticated development than I believe NK is capable of.
KN,
Yes, that's good logic. I agree that China isn't a good nation and they do things to their own citizens that would never happen here or in most of the west, but they normally make decisions that are based on the good of China, or at least their idea of what's good for the state. If this is tied to China it would not be good for China. That said, your analysis is sound.