The stance taken by the North Koreans needs to be looked at in its historical context. The largely American UN intervention in Korea in the Korean War led to the destruction of much of the infrastructure and a huge civilian death toll. The bombing campaign was more intense than anything seen in WW2. Le May estimated 20% of the north korean population was killed. Dean Rusk did not want to leave one brick on top of another. The use of nuclear weapons was threatened and actively sought by Le May.
It is little surprise that the north koreans would do everything in their power to prevent a repeat of what happened in the early fifties.The continual bluster needs to be replaced by both sides thrashing out a peaceful resolution. The present bluff and counter bluff could spiral out of control.