To keep N. Korea in its orbit. This is not about nuclear-free prosperous N. Korea. What they fear is N. Korea veering off from China.Going neutral is also unacceptable. If they can make N. Korea as another province of China, then they would give more help.
What they would like to achieve is to make him ineffective.
China is not ideologically committed to NK, strategic aims aside, and sees no profit in an economical basket case, a holdover from Stalinism.