Supposedly, the nuclear weapons that N. Korea are in possession of currently aren't capable of reaching nations that are great distances away from them (U.S). However, the nuclear weapons easily pose a great threat to S. Korea and, according to S. Korea, could easily destroy major S. Korean cities, especially Seoul.
The only reason North Korea's nuclear capabilities are becoming a bigger threat is because of the volatile people in charge of the whole operation. Ill-tempered and sensitive people should not have this kind of power. It is dangerous because they are willing to cause destruction of the whole world just to have power.
There are many more people knowledgeable about nuclear weapons these days and the necessary materials for producing these weapons of mass destruction are more readily available. The economy of North Korea is strong enough for them to devote more funds to development of nuclear weapons, even though they do not adequately provide for civilians.
Yes, the threat of regime change or revolution in North Korea becomes greater by the year. The country's people are starving, and the regime has maintained its oppressive human rights antics. North Korea's nuclear capabilities could fall into the hands of even more incompetent persons than the current regime. Instability in North Korea could be detrimental to the world because of the risk of nuclear weapons falling into dangerous hands.
North Korea's primary goal is to oppress their own people. The leaders of North Korea just want internal power. There is no current sign that they intend to use nuclear weapons on other countries. Until they make an overt nuclear threat, there is no reason to believe that they intent to hurt the world with nuclear weapons.