You are making some assumptions there. There is no guarantee that China would jump in and WWIII would kick off.
I don't see how you can let a crazed dictator threaten to nuke you off the face of the Earth and then demonstrate that he has the capability to do it.
Maybe you are ok with getting vaporized, I am not.
NK has only made threats in response to US threats.
you are underestimating the strategic value of NK to china. china is NKs financer with something like 90% of all Nk trade done with china . if the US attacks NK, NK will respond (at the very least) by attacking US bases in SK starting a korean war.
the US already has something like 400 military bases surrounding china. a capitulation of NK would mean tens of thousands of US troops along with huge military installations right on china's doorstep. it's highly unlikely china is going to just sit back and let that happen.