Honestly missiles are not a threat to S Korea. Position several C-RAMs and anti ballistics and S. Korea will defend themselves by just attrition. (S. Korea has the economy buy the resources and the US moving THAADs into defensive position before they can hit any population zones while NK has a limited stock of missiles) Let alone Camp Casey, Camp Stanley, (several other,) and the 7th Fleet there for support. The biggest threat from N. Korea are actually conventional artillery. Weapon of choice for the N Koreans is the Koksan, a weapon system that can fire 170mm rounds with a maximum effective range of 60 km (which so happens to be near the distance of Seoul from the DMZ) With a population of 10 million and how S. Korea is build (very compact and upwards) the initial salvo will have casualties in the upper hundred thousands if not close to million. The only way one would counter an artillery barrage is monitoring the DMZ for movements and make a preemptive strike on the sites. NK actually has the means currently to do massive damage to Seoul and they don't need any missiles to cause a panic.
Field artillery is capable of reaching South Korea if they shoot from just across the border, but the intent of tactical ballistic missiles would presumably be to pose a threat to South Korea from dispersed locations deep within the country. I'm sure they are presuming that in an actual shooting war with South Korea and presumably the US, their above ground assets near the DMZ aren't going to be around for long.
I see some analogs to how Saddam Hussein viewed his own military, but even more extreme. North Korea's military exists not to defend the country and not even to win a war against another country. It exists to provide credibility to the assertion that North Korea is a legitimate threat that has to be taken seriously. Saddam Hussein faced real enemies with real armies that required his military to be able to fight real wars (notably: Iran). Realistically, North Korea is never going to face a similar regional conflict, and I suspect they know that.
The reason North Korea has intermediate ballistic missiles is because North Korea wants to say they have intermediate ballistic missiles. The reason why they are working on intercontinental launch platforms is because North Korea wants to say they have intercontinental missiles. You can never fully predict what crazy people will do, but North Korea makes a lot more sense to me when I imagine the country run by a sixteen year old that wants to convince everyone he knows Kung Fu so no one should mess with him.