By supporting this despotic regime for more than half a century, the Chinese will have to accept the deployment of a (more advanced) missile defense system (THAAD). While I'm sure they're going to retaliate against the South Koreans (and America?) possibly through a boycott and diplomatic sanctions, the South Koreans may have no choice but to try to improve their defenses. If you were faced with a nuclear attack wouldn't you be willing to suffer a bit economically in order to get a better defense?
Of course if the Chinese push too far or the North Koreans convince the South that their defenses are useless then we may see the worst possible outcome for the Chinese (and probably everyone); South Korea will build the bomb. This is different from the U.S. having some tactical nukes place in South Korea but presumably under control of the U.S.; a S. Korean nuke will make it very clear to North Korea that if they bomb Seoul that North Korea will be completely and utterly wiped out. It won't matter to the South if the North can strike the U.S., they won't care. Of course there's a very good chance that once both Koreas have the bomb that Japan will quickly follow suit, 2000 years of animosity isn't forgotten that easily. This will greatly complicate China's domination of Asia because they'll always have to worry about a catastrophic (even if suicidal) conflict with their formerly vassal states. For example, if China and Japan then got into a serious dispute over the Sendoku islands the whole world would hold its breath.
One of the reasons why this is coming to a head now is that North Korea realizes that South Korea is without an effective government. The president has been impeached and is awaiting a ruling from the high court to make it official. In the meantime, the interim leader doesn't have the political capital to make big decisions without the mandate of an election. So North Korea is pushing and pushing and is trying to see what it can get. Unfortunately for them (and everyone) the only person they could negotiate with is someone who's grasp of the truth is tenuous and he is erratic to put it mildly (especially at 3am, twitter time). So there's a giant game of chicken being played blindfolded.
If the American missile defenses were reliable then perhaps this could all be avoided; the North Koreans could threaten all they want but a nuclear warhead couldn't make it to American soil. This was the essence of the American position; the Americans realized it's much harder to intercept short range missile/cruise missile attacks traveling the 50km from North Korea to downtown Seoul (2 min. flight time). So despite the nice visuals of Patriot missiles hitting Scuds, the U.S. told the South that when (not if) North Korea could nuke Seoul, we'd retaliate for you with our nukes. That capability kept the North from having a real threat. Now however, the thought that the U.S. would retaliate for South Korea becomes less credible when North Korea can then (in theory) take out Los Angeles or Washington D.C. Everyone realizes this, so if the North gains a credible ICBM capability and if the U.S. lacks a credible defense, the American guarantee is gone and South Korea is left to the wolves (North Korea). Hence the panic over the inadequacies of the American missile defense/desire for better local defense.
Returning the subject of this article, the reason why the American defenses don't work (reliably) comes down to simple physics. My physics professor at Harvard was one of the ten(?) members of the scientific commission evaluating Ronald Reagan's "Star Wars" missile defense system. (Having been given top secret clearance he told me about a trip he had to the Groton, Connecticut sub base into the bowels of one of the "boomers" (SLBM subs) where he was led into a giant space with huge tubes running from floor to ceiling. The admiral escorting him turned to him and said, "Professor Horowitz, you're in the same room as a 200 H-bombs". He told me that basically the problem of hitting a bullet with a bullet greatly understates the problem; the bullet is painted black against black, is maybe on an erratic trajectory, maybe has decoys, maybe there are nuclear explosions nearby interfering with your sensors, at a time when you are likely to be least ready. All of this in a volume of millions of cubic kilometers in SPACE far from your command and control systems.
Oh and while the cost of the attacker's missile might be say 10M dollars; the potential damage (if it can get through) could be in the TRILLIONS. There's a lot of bang for the buck so to speak (which is why nuclear weapons were developed of course) which is why the attacker is sparing no expense to make as many of them as they can. Only one has to get through. So while computers and sensors have improved by many orders of magnitude since Reagan, it's still a very hard problem, and it is a problem in which a 56% success rate just isn't remotely good enough.
So because of physics and despite the hundreds of billions spent, my professor was then and is still right; anti ballistic missile defenses are not good enough against a nuclear attack where even a few (or one) failure may mean millions will die. The Chinese, by coddling and aiding and abetting a "genocidal" regime (can you call it that when it has attacked its own people?) had many many opportunities to alter the course of this tragic outcome. That they did not is a stain (from the blood of millions) on their hardly unblemished reputation.
Soap box, ballot box, jury box, router box, ammo box, TATA box- (With apologies to Stephen Decatur Miller, may he rest in peace). Updated for the 21st century, router box is BEFORE ammo box because while a cyberattack may cause more damage than a mere bullet, it will likely put the patriot at less risk (so use it first!). Of course the TATA box (which may soon be within reach of many) may put EVERYONE at risk but it changes the expression "Give me liberty or give us (meaning maybe everyone) death"