The interests of China and North Korea have been steadily diverging. To start with China has learned that it can live beside a powerful united pro American Korea as a natural ally. China itself is powerful enough to see off any such challenge in the event. Better, the probability of such a war will be of China's choosing.
Why China still supports North Korea, in six little words
Although the Chinese do not view North Korea’s acquisition of nuclear weapons as an existential threat to China, there are worries that a perception of a growing nuclear threat could lead South Korea, Japan, and even Taiwan to develop nuclear capabilities. North Korea’s nuclear programs and demonstrated provocations have already prompted the United States, Japan, and South Korea to strengthen defense coordination and have led Tokyo and Seoul to enhance their missile defense. These developments are judged to have had a harmful impact on China’s security environment.