The leader of North Korea are so maniacal. I do not believe they value human life the same way that we value human life in the United States. For this reason, they are a threat. When human life does not matter, it is an easy decision for someone to take human life.
North Korea makes numerous claims that it has the nuclear WMD stockpile to blow the United States out of the water every other month, and even makes the bold claims that it's tested these devices successfully either within their own borders or at remote locations out to sea surrounding their nation. They're just blowing steam and trying to act tough to get more attention.
No, I do not think that North Korea is much of a nuclear threat to the United States at this moment. Based on seismograph readings, the bomb that the North Koreans detonated last week was barely as large as the atomic bombs the USA used on the Japanese to help end World War II in 1945. And this would mean they only now have the capability to develop them, not even produce them on a scale nearly as well as the USA has.
North Korea may have a large army, nuclear weapons and an unhinged dictator, but they are not a legitimate threat to the United States.
Why? Because he is a dictator and North Korea is a totalitarian state. To treat them as an actual threat, gives their system legitimacy when it is based on the subjugation and suffering of their people.