China is not the absolute master of North Korea. China helped preserve the North with its intervention into the war in 1950, but the Kim dynasty generally does what it wants, often in defiance of China. China's fault is that it keeps sending enough support to keep NK breathing, however weakly. As in 1950, China wants a divided Korea with a weak north. This means maintaining the North, but by doing that they are stuck with an irrational and dangerous neighbor.
Just as the Cold War "Communist Bloc" was not a unified monolith, North Korea is not identical to China. But, China needs to realize that the North will eventually fall, and they need a policy to deal with that, instead of delaying the inevitable.