this thread got weird :p

but I don't think China is as concerned about the security issues as much as it is about the humanitarian and economic ones. If NK collapses, who is going to see an influx of refugees? China.

Who is going to need to bail NK out after they get completely obliterated? China.

What if in a last ditch effort to screw the enemy, NK decides to launch a nuke at Seoul, and fire one at Japan? That's going to really mess up China's standing in the world of they continue to back NK.