China and South Korea Fear a North Korea Regime Collapse

I am reading in an article in the San Francisco Chronicle by Matthew B. Stannard as a hat tip that a Hoover Institute Fellow is quoted in the article. The article is about being a little stunned that China has rebuked North Korea and has called sanctions.

Stannard’s basic conclusion is that China version of sanctions may be quite different from America’s version. The usual reasons are offered such as: close economic and military ties to China. Basically North Korea’s survival is due to the economic bones tossed by China to its rascal little puppy.

Here is the thing that tossed me into the wonder zone:

For all its anger at the North, say analysts, China may not be ready to support the kind of broad economic and political sanctions that stand the best chance of influencing Pyongyang, but that risk causing the North Korean government to collapse -- exposing China to a flood of North Korean refugees.
…

"If we use sanctions that are serious sanctions, it would lead to the regime falling, and that's the nightmare of China and South Korea," said Charles Hill, a former special consultant on policy to the secretary-general of the United Nations, now a research fellow at the Hoover Institution.

As to the first paragraph, I am fairly certain that China could utilize the world’s largest manned army to prevent a flood of North Korean refugees in the event of a regime collapse. There are a few paragraphs to read using comparative illustrations to click on an American brain, and then we come to the paragraph about China and South Korea.

It is implies that China and South Korea are buddies and that there is a mutual dread of a North Korean collapse. Now I can see why a North Korean collapse might be a bit uncomfortable for China, however someone needs to enlighten me as to why the South Koreans have the same dread.

Do the South Koreans fear a North Korean regime collapse will automatically result in a nuclear strike in South Korea?

I am thinking a North Korean collapse might lead to a re-unification of a ONE Korea. I think China’s new global confidence can live with a unified Korea as an incredibly viable trading partner. A unified Korea would lead to fewer tensions in that part of Asia. It would give America a chance to lessen its military presence and enable China greater influence as a hegemon on mainland Asia.

The only thing China would need to get over is a tiny mud on their ego of another failed Marxist dictatorship.

I suspect that with North Korea gone, Japan and a New Korea bloom economically as partners. Now that may not necessarily be in the best National Interests of America, however it would go a long way in America being freed up to concentrate on the War on Terrorism in the Middle East.
Taiwan may even benefit from Little Kim ceasing to exist as a saber rattler.

With Korea and Japan cooperating with China, a capitalistic Taiwan will want some of that market stuff too. It would be to China’s National Interests to use Taiwan as a distribution center to the Western world for their cheap labor manufactured goods.

So again I wonder: What is South Korea’s fear?