North Korea is actually a bigger threat to China than to the U.S.

Why?

Because if North Korea gets too strong of nuclear weapons it makes China look weak that they cannot stop this.

It makes China look like a paper tiger.

The U.S. is not a paper tiger with Trump in office. If he promises to do something often he does it!

But, if China is perceived as a paper tiger around North Korea then China's world stature as a nation is threatened.

So, following this logic North Korea threatens China's stability more than the U.S. or Japan or South Korea.

For China though there is no GOOD solution. If they counter Kim jong Un then what happens to north korea? Does north Korea become a part of China with South Korea on it's border?

This likely would be unacceptable for China to have a democracy and a relatively effective democracy on it's border directly.

So, there likely is no good solution for China. But, if China does nothing China will only look weak and vulnerable in the eyes of the whole world.

So, from this point of view North Korea becomes China's biggest enemy on earth at this point in history.