I think it would be interesting to know what North Korea has on China that would keep China from responding.
If North Korea is used by China to accomplish illegal trade or foreign policy goals then North Korea could probably get away with a great deal.
North Korea only exists because of China so there must be a benefit flowing to China.
Yes, to China, N. Korea is a dagger to bleed U.S. and also a shield to protect itself from U.S.