The main reason is that China doesn't want to compete with a unified Korea. They want to be the next dominant super-power without rival and if supporting an evil dictator is necessary, they'll do it. The odds of a nuclear war with NK are diminishing with time...5'7" and over 300 lbs...sounds like a recipe for a walking heart-attack.
Not likely, unless a direct and imminent threat to the US is perceived. Everyone knows Seoul would be completely destroyed.
The CIA did a study a few decades back of what they would do if they were the dictator of North Korea. They all agreed their only real move was to get nuclear armed at any expense. Some nuclear missile experts have been closely watching the progress North Korea is making - they built a life size replica based on photos taken from space. They are astonished at the leaps they have made in relation to their resources. Kim J. is infatuated with the American life style - he would never want to harm his best friend Dennis Rodman whom lives in the U.S..
In one sense, the best way to get rid of NK's nuclear missiles would be to induce Kim to fire them off. The "best" targets for the purpose would be the most distant - this allows longer time for interception, less accuracy in targeting and longer time for retaliation before a second wave of missiles could be prepared. Targets on the US mainland would be, I'm sorry to say, "ideal". Which makes it pretty certain that Kim would not be so stupid.
Population of North and South Korea - 77 million Population of China - 1.38 billion GDP of South Korea - $1.4 trillion GDP of China - $11.2 trillion The leaders of China are hyper concerned about stability. It is at the root of everything the authoritarian government does. A war and subsequent refugee crisis on their southern border, coupled with the chance of contagion of that instability to other parts of China, are their primary and almost pathological concern. Worrying economically about a country that is more than an order of magnitude smaller in population and GDP is the least of their concerns. And militarily a unified Korea would have a significantly smaller military footprint than the current two entities, with the added benefit that the U.S. loses it's excuse to keep 25,000 U.S. troops and the weapons for significantly more permanently stockpiled on the Korean peninsula.
China also has less worries with their military then most ( all ? ) countries because you can easily conscript a huge army from their massive population that they control rather tightly. The US has the weapons but I doubt millions of Americans want to sacrifice themselves in a senseless war, especially after seeing how senseless the invasion of Iraq was. The worst thing the US can do for everybody is start a war on some preemptive bs basis like Iraq. China will change their position if North Korea goes rogue in a meaningful destructive way.
Trump wants to stir the pot: it excites his base, and the defense manufacturers are his donors. Likelihood of US first-striking NK is zero or close to zero. US will not do it unless short fat man does it first, and I doubt he's that stupid. If NK strikes US, US will annihilate NK. This is, and has been US policy in recent years: never the first strike, but a guarantee to retaliate in kind. Best way for US to deal with Kim is work with China/South Korea to effect an internal coup. (We are likely already doing this.) China may not want a reunification or a democratic neighbor, but I'll bet they'd be comfortable with a less volatile dictator who is more intent on feeding his people than making bombs. It would be profitable for China and South Korea to have a market and not to have to worry about war, refugees, heavily guarded borders, and starving, desperate people sneaking in. And I'm sure there are plenty of members of the elite in NK who would love to off Kim and take his place. Getting rid of Kim in a relatively peaceful way and replacing him with someone who wants to create a real country would be a win for everyone--especially the very unfortunate people of NK who are the main ones getting the shaft.
I agree that China is likely concerned about a refugee crisis as well as a possible nuclear war breaking out in their backyard. I've often wondered what would happen if one of the fat boy's rockets veered off course and landed in Beijing. China is not building artificial islands with military bases and runways in an attempt to improve stability in the region. They are doing that because they want to become the next military super-power or at least equal to the US. China actually proposed brokering a deal where NK would stop nuclear testing in exchange for US leaving the region. How could China suggest that if they were not already aligned with / had influence over NK? If China really wanted to put more pressure on NK, they easily could. China is NK's primary trading partner (around 83% of all NK trade). So why haven't they? I also think South Korea could gain a lot if they could overthrow the NK dictator and merge as one democratic country. If that happened, the unified Korea would have over double the land mass (and natural resources) as SK and I think NK people (after some time) could be a huge asset to growth. So although SK is today around where China was in 2002 in terms of GDP, that GDP is despite NK. I think a combined SK + (democratic) NK GDP would be much higher. I think China realizes that too.
It is strange, isn't it. I wonder what Rodman thinks about some of the things Kim said about Obama regarding Obama's ancestry. http://www.telegraph.co.uk/news/wor...un-lambasts-Obama-in-bitter-war-of-words.html