Nothing short of armed conflict will resolve North Korea's nuclear program. China will never allow NK to collapse as long as the US has 30,000 troops in South Korea and 20,000 more in Japan. Having a "crazy leader" that appears capable of doing anything in their pocket can be a useful tool to keep adversaries at bay. Its worked well for them so far. 
A reunified Korea with the US as an ally could also challenge China's dominance of the region and cause great instability within China itself. The Communists greatest fear is that they'll be overthrown by their own people. It's really just a matter of time.