The DPRK has many violent foreign enemies that they need to protect their small nation from. If their enemies are allowed to own a massive nuclear arsenal what is the major issue with allowing them to have something to equal the playing field. The DPRK have always said that the nuclear arsenal will be used for defense issues only if the country is a attacked, isn't that the same thing the United States and its allies say about their arsenal?
Total destruction is what nuclear weapons can bring. Imagine if every country has a nuclear weapon and use it against other countries, then the response was also a nuclear attack, human extinction comes into the picture. A country like North Korea who threatens US with a nuclear attack is clearly not fit to have that kind of weapon. Not because other countries have nuclear weapons that all countries should also have. It's not about equality but practicality.