You do realize they could annihilate South Korea in a moments notice, correct?
No, they can't. It would take numerous well targeted nuclear warheads to annihilte the South Korean cities due to the mountains that cover the majority of the peninsula. The North Koreans just do not have the delivery systems and warheads to accomplish the task. The mountains also provide protection against conventional artillery as they block the flight paths of the projectiles.
The North Koreans have built a shock army based on Soviet doctrine of achieving a breakthrough of the main line of defense and exploitation with motorized/mechanized forces to capture strategic objectives and surround defenders. The North Koreans know that their window of opportunity for a successful invasion is pretty short, and they know that their artillery must be targeted against the defenders at their main points of attack. So using their artillery to bombard Seoul is not a winning strategy for the North Koreans either.
The only way for the Norht Koreans to annihilate the South Koreans would be to defeat them in ground combat, and that just isn't going to happen.