North Korea's nuclear program will lead to further proliferation unless they are taken out

NY Times:

Allies Weigh Nuclear Options as North Korea Threat Looms

• As the North races to build a weapon that could threaten American cities, Japan and South Korea are debating whether they need their own nuclear arsenals.
• Some worry the United States might hesitate to defend the countries if doing so could provoke a missile launched from the North at Los Angeles or Washington.

It will be difficult to keep Japan and South Korea who are both stronger technology wise than North Korea from building their own nuclear weapons if North Korea is not neutralized and denuclearized.  Both have significantly stronger economies.  They both have companies that are stronger financially than North Korea. 

This reality gives China more incentive to do something to tame North Korea's nuclear ambition.  I feel certain China is aware of that possibility.  That is probably one of the reasons they have backed the sanction regime so far.  They may have to do more.

If North Korea provokes a US attack China may be put in the position of joining in that attack if it wants to avoid having US and South Korean troops on its doorsteps.

There will probably come a time when the US will need to launch a preemptive attack.  Failing to do so would be gambling with the lives in West Coast cities or possibly a devastating EMP attack which could cripple much of the country.  At North Korea's current stage of development, it would be better to hit its ICBM's in transit to a launch point than to gamble on missile defense.