One response to NK has been discussions of introducing THAAD ABMs into SK. China complained about this, because the THAAD missiles could be aimed at Chinese missiles.
I have to ask myself, why would Chinese missiles be flying around the area in the first place, if not to attack SK?
Just sell them ours.
To attack Japan - or threaten to. China needs the "threat" of Japanese aggression to take their public's mind off the impossibility of maintaining 7% growth and the inevitable economic decline they are facing. US-operated THAAD systems in South Korea pose a threat - however slight - to their ability to threaten an attack on Japan as a diversion to help keep the Communist Party in power.
China has zero interest in a military confrontation with South Korea - indeed, they are now likely to support the South in any hot conflict with the North - but South Korea is also right in the way of any missile-rattling they might like to engage in against their ancient adversary.