What is the worst scenario possible in this case? Let's say the the DPRK regime just throw their hands in the air and decides to simple wipe out ROK from the map, and so launch a kamikaze offensive against them with the objective of causing as much damage humanly possible before being defeated, what could they achieve?
Max? No one can say for sure, open source doesn't give enough facts (unlike say the U.S. and Russian ICBM nuclear stockpiles, both of which are literally known to the launcher), but some reasonable speculation would include (this is based on the North having five launchable nuclear weapons, 3,000 tons of Vx and a similar inventory of mustard gas or Lewisite and the North going gfull out Zerg-rush):
Destroy Seoul, or at least make it uninhabitable.
This would take around 100 240mm/300mm MLRS rockets with a mix of Vx and Mustard gas combined with a conventional bombardment (the idea is go conventional artillery 1st, that forces people into shelters, many of those lack full NBC protection, followed almost immediately by the CW agents. Both Vx and Mustard are heavier than air, so the agents tend to gather most strongly in basements, dugouts, and bunkers. Vx is incredibly lethal, 10 mg on the skin is lethal in half of all cases, 30 mg is L50 if inhaled. It is also extremely persistent, unlike earlier agents like Sarin. Mustard has a much lower lethality rate, but that is part of its advantage in warfare. Individuals who are exposed require serious and ongoing medical care, often for the rest of their lives, blindness is not uncommon. This quickly overwhelms the medical triage system.
This could also be achieved with a salted nuclear weapon. That would render the region unsafe for up to 20 years.
Destroy or contaminate other major Korean cities. The North has 200+ missiles that can strike any point in South Korea from north of Pyongyang, including at least 100 that can manage it from the Yalu. There are roughly 20 million (40%) ROK citizens who live within 75 miles of the DMZ, they are all within range of artillery rockets like the FRoG-7
Destroy major industrial sites or contaminate them. The DPRK had 600+ SRBM that can carry conventional, chemical or nuclear warheads (although it is unlikely that the North would waste one of its few devices on a simple industrial target).
Now for the really "special" possibilities.
Strike metro Tokyo with one or more nuclear Weapons, other possible targets include Kyoto, Nagoya, and Osaka
Strike Kadena AFB, Okinawa. This is at the outer edge of known DPRK missile range, but it is also a glorious target for a lunatic who wants to see the world burn.
Top number casualties, if things broke perfectly wrong? Just from Pyongyang's actions? 15 million, maybe a few million more once short term radiation effect is taken into consideration, 90% civilians. More likely would be 3-5 million, 50% lethal. If the North managed to get a WMD onto Yokosura Joint Base or Kadena AFB, both of which have THOUSANDS of U.S. dependents on site, Katie bar the door. Under the previous Administration I would have expected a response that would scare the crap out of everyone who ever serious considered using a WMD against U.S. civilians.