Yes, I believe that North Korea's nuclear program is a threat to global security because the country's leader is very volatile and aggressive. North Korea have constantly threatened to launch nuclear missiles at their enemies, hence allowing North Korea to grow their nuclear arsenal is a threat to the world's safety.
China has made their decision on nuclear-use technology based on their own criteria. There is surely a reason for it, logically. The world should know why they have made that determination, and if it's valid, then other countries should make their own decision on it. But it isn't for us to say if they are a threat to global security without understanding the situation.
Any country in the world who has stock piled nuclear weapons, or is in the process of developing these weapons, could be considered a threat against world safety. I think North Korea in particular is a threat because they have shown that they are willing to make what we consider irrational decisions and may use these weapons without a good reason.
The so-called "Hermit Nation" of North Korea has often made a big deal about their nuclear missile testing, but all these tests have so far failed. I therefore don't think that their nuclear program is that much of a threat. China may believe that they are, given that they won't supply them nuclear-related technology, but other nations disagree and won't follow suit.