I think people are underestimating how difficult it is to destroy a city the size of Seoul. It's a vast megalopolis of 20 million people in total in the metropolitan area. Even North Korea's large long-range artillery park would only cause mild damage on the whole.
It depends, in large part, on how you are defining "destroyed".
It is quite difficult to recreate 1951 Seoul (or 1943 Stalingrad). It is not so difficult to render the entire region uninhabitable. We have already discussed CW and actual nuclear weapons. There are, however, two other, very cost effective, methods that can be used, namely bio-weapons and "dirty" bombs.
Pyongyang has demonstrated a truly remarkable disdain for international norms, far beyond even the late and unlamented USSR on its worst day. The USSR had (and Russia, as has been documented, continued, although current status is not open source available) a large scale, thriving bio-weapons program long after the 1972 Treaty, as did Iraq. Iraq is particularly worrisome as an example since Saddam's government was vastly less efficient in the area of weapon research than the DPRK. It would actually be far MORE surprising to learn that the North does NOT have a BW program than to find it does. A relatively low number of Anthrax laden shells or warheads could render significant areas of the Seoul metroplex a no-go zone for years, if not decades.There are a number of other agents that could be produced by any competent lab (and the DPRK has demonstrated that it can gather together some very competent people), and be done with mimimal chance of accidental discovery.
A "dirty bomb" would generally not be seen as a serious threat, compared even to conventional HE/Frag shells. The difference when considering the DPRK is that there is no question that the North can and has produced considerable amounts of Plutonium and is readily capable of producing Polonium-210 in quantity. Not only would the release of any appreciable amount of plutonium oxide or Po-210 dust into a region present an actual threat it would also result in the evacuation of an area larger than that which had actually been exposed. Residents of the region, or abutting regions would also be reluctant to return, even after the ROK government indicated that it was safe. Loss of real estate value alone would likely reach 10 figures.
There are very few nation-states that would consider use of bio-weapons or heavily radioactive dirty bombs close to their border (or on the same continent), Pyongyang's leadership is one of the exceptions.