China Needs Unstable North Korea

There are a few articles like this one, by Matthew Lee and Christopher Bodeen, US, China Spar Over North Korea, South China Sea http://abcnews.go.com/International/wireStory/us-china-spar-north-korea-south-china-sea-36541614 that describe the tension between the United States and it allies in the area around the South China Sea.  The Chinese have not stopped doing what they said they were going to do there, and there is little consensus on how to stop them.  North Korea has a part to play in this, and it is a part that the Chinese need.
 
China controls North Korea, but says it is like a wayward son, out of control at times and not listening to its parents.  That is exactly the image of North Korea that China wants the world to have. It has only been recently that the Obama Administration has tried to correct that image and put pressure on the leadership to do something about the son.  This is a kid that runs around the neighborhood killing people and bullying where he can get away with it.  The parents let him get away with it.

North Korea openly threatened the United States several times, a threat we take semi-seriously some times.  They have actually sunk a South Korean ship and fired artillery into disputed South Korean territories.  They recently tested another nuclear bomb of some type - and we know they have the delivery mechanisms for that bomb to reach some parts of the US.  It really doesn't matter what parts.  These are big bombs that will kill hundreds of thousands and do terrific damage.  They just have to hit in the neighborhoods of any city.  CNN says Iran showed up at the last tests so we get the idea that  the kid is bringing in gang members from other neighborhoods and building a base.  

The Chinese need North Korea because the blame for these activities falls on them, not China.  But, it is China that benefits.  It is almost like the kids are building up a gang that terrorizes the surrounding town, while the parents sell security services to the community.   We are tired of this, at last.  Why did it take so long?