China has a vested interest in its North Korean proxy being able to threaten the US and its allies. Do you suppose South Korea is any competition for Chinese industrial strength ?  Do you suppose North Korean first strike potential is an advantage to the  Chinese military strategy ? Do you suppose the Chinese are happy when a US carrier fleet shows up off shore North Korea for military exercises ? This whole little Korea passive aggressive BS needs to be understood for what it is. Just a cold war between the US and China