Why would China want to annex N. Korea? Whats in this for them?
Primarily: stabilization of the tensions within the region (war and such aren't good for business - and China has gotten very good at business).
China and North Korea are allies, but China knows North Korea's leadership is a bit crazy. While they may officially support them, every time N Korea does something like this you can bet China is gritting it's teeth and sending them a private nastygram "suggesting" that they behave themselves.
Not to mention the simple fact that North Korea really has no significant allies outside of China. Nobody really gives a darn about them. If China said tomorrow that they were going to invade North Korea the world would pretty much collectively shrug their shoulders and say "Go for it". Heck they may even view it as a nice training exercise for the Chinese army.
