Some links on China getting sick of NK's shit:
http://www.reuters.com/article/us-china-northkorea-coal-idUSKBN17N12A
http://www.reuters.com/article/us-northkorea-nuclear-un-idUSKBN13I1OY
Kim Jong Un executing his uncle:
http://www.bbc.com/news/world-asia-25359939
From the article:
"In August 2012, Mr Chang made a high profile trip to China, where he met then-President Hu Jintao. The two sides later signed a raft of economic deals, including the development of two special economic zones: Rason, on North Korea's east coast, and Hwanggumphyong, on the border with China."
Kim eliminated somebody that had a known, productive, relationship with China.
Kim Jong Un has purged over 340 people since taking power:
http://www.cnn.com/2016/12/29/asia/kim-jong-un-executions/
"Of those killed, about 140 were senior officers in the country's government, military and ruling Korean Worker's Party."
The key word is "senior". These were people who had been around a long time and were known to have relationships with the Chinese. How do we know? Because 85-95% of NK's economy is based on trade with China and China is NK's main source of food, energy and arms. Everybody in NK's government has had to deal with the Chinese at some point. Senior personnel have had a longer relationship with the Chinese government; their elimination limits China's resources in Pyongyang.
Piece on the deterioration of China-NK relations:
http://thediplomat.com/2017/04/the-...peoples-republics-chinas-north-korea-problem/