I'm sorry, but I really have to very, very strenuously disagree with your interpretation of China's role in North Korea's nuclear program and its goals for political positioning in the coming decades.

China’s policy towards North Korean nuclear proliferation is guided by a desire to protect the territorial integrity of China, to maintain China’s position as the regional leader on the Asian mainland, to preserve international prestige, to preserve the historic friendship between the DPRK and the PRC, to counter the perceived US containment policy, and to protect Chinese trade interests in the region. Any escalation on the Korean peninsula is likely to create a refugee crisis within China as North Koreans flee across the Yalu River into Jilin and Liaoning provinces, giving China a territorial imperative to help prevent any such escalation from occurring.

China officially supports the concept of a multi-polar world order, it sees its own diplomatic efforts to limit North Korea’s nuclear program as a way of limiting the need for US involvement in a neighboring country, and as a way of preserving the buffer between China and the ROK, which houses tens of thousands of American armed forces.

As the top power on the Asian mainland, and as a nation interested in being the leader in the region’s affairs, China wishes to be able to demonstrate an ability to serve as an arbiter for disputes among its neighbors.

In its attempts to keep the Korean nuclear issue a regional affair by keeping it out of the UN, China has ironically been forced to increase the US’s role in discussions. However, China has employed its own form of coercion with North Korea, as the DPRK remains highly dependent on Chinese oil and food transfers. In 1997 China threatened to withhold food aid over a diplomatic spat with Kim Jong-Il, demanding that North Korea engage in agricultural reforms. Chinese food aid to North Korea decreased steadily over the first five years of the 1990s during the nuclear crisis, but increased again during the latter part of the decade. The North’s dependence on oil from China has likewise been used coercively, with China shutting down a pipeline to North Korea for three days in early 2003 during another DPRK-US “diplomatic showdown.”

One of the key concerns China has with a nuclear North Korea is that the development of a Korean bomb will spur other countries in Asia to engage in an arms buildup and to develop nuclear programs of their own, potentially weakening China’s regional hegemony. Japan might seek to rebuild its military capacity if faced with the potential of a North Korean attack, and South Korea has, it revealed in 2004, maintained a clandestine nuclear program (Washington Post, 9/12/04). North Korean advances in missile technology, culminating in the 1998 firing of a ballistic missile over Japan, have fueled concerns in the US that missile defense systems should be installed both domestically and to protect Japan and South Korea, a move which China says would force it to begin stockpiling additional nuclear missiles of its own, in order to ensure a large enough arsenal to overwhelm any missile defense system.

Viewed through a proper lens, what China is doing is both consistent with past policy and perfectly in line with their regional goals. There's nothing untrustworthy about a country behaving exactly as it has in the past and in a manner consistent with its stated goals.

Regardless of China, you ought realize that putting sanctions on a country such as N. Korea will not hurt Kim Jumg Ill, but will only serve to starve the commonfolk of that country, as him and his regime care little of them.