The problem is that the US does not care about potential human disaster in the event of a major war in the Korean Peninsula; China does care about the humanitarian situation.
Maybe the US don't care the human disaster but South Korea does. So things is not that simple that US will bomb DPRK because of hatred of Kim Jing un.
Sometimes you are in dilemma.
Attacking DPRK may cause a humanitarian crisis , don't do that it seems there's no other to stop the development of DPRK nuke program.
Compared to the humanitarian crisis, the nuclear attack is more horrible for South Korea, and also for China.
An form of existence is better than a war in the Korean Peninsula, and China does not want war. Hence, it tries to slow down North Korean nuclear development on the one hand, and, on the other, it tries to instill some common sense in the South Korean hawks.
Nobody wants war and nobody wants a crazy neighbor with nukes.
China has been trying to persuade DPRK to give up the programs since a long time ago.
The world has always thought that China is the only one who has a great giant influence on DPRK, cause China fought for him and the 90% export goods of North Korea is transported to China and Its nearly 100% import comes from China. But China still have no influence on DPRK which is totally controlled by Kim Jung un and his followers.
As for now, China failed on stopping the nuclear and missile program.
So Trump said either you stop him or I will do it.
This makes China in a big dilemma.
China has no idea for changing the mind of Kim Jung un and China also couldn't do more thing than implements the embargo which still don't work.
Let's assume China succeed in stopping the US's attack towards DPRK just as the past years when the nuclear crisis happened.
What will happen?
First, DPRK will make its nukes practical, then SOK feel horrible and want nukes and missile defense system.So do Japan.
In the end, China will be surrounded by nukes. This situation is definitely unacceptable to China.
Even feeding total 20 million hungry North Korean which is less than the population of Shanghai is better than being in that situation.
There is other solution offered and mostly excepted by SOK ,which is China take off Kim Jung un and stablise DPRK.
But it means a deviation from our nonintervention principle that is the main basestone of foreign policy.
He is mad. And now we are another mad man on the other side of the ocean. That's explosive.
China will never be trapped by two mad men, one of whom the other day proved in Syria that he could really act like a blood-thirsty third world dictator.
Don't overestimate the power of US president. The decision of attacking Syria is the will of establishment camp.
You can watch the news of American mainstream medias, which bravo Trump for the attack to Syria loudly for the first time since Trump moved into White House.
So you know who is in charge of USA. Not the man living in White House. It is the establishment camp.
But in DPRK, Kim Jung un does control everyone and everything.