China will try to restrain DPRK and may even veto US Security Council action. However, a veto would not prevent ROK from acting (as it already has) unilaterally -- and the US would be dragged into any shooting war. The US has in the past prevented ROK from acting against DPRK and indeed talked ROK out of a nuclear weapons program (all US nukes were removed from the Korean pennisula by 1991) by pointing out that ROK was covered by US nukes deliverable by missile and bomber. What the US (and China, Russia, and Japan) fear is a unilateral action by either side in Korea. ROK will probably not attack DPRK, but the same cannot be said of DPRK. A dying regime may want to take whoever it can with it.