They don't plan to reunify, they plan to conquer. And I'm sure it hasn't escaped notice that both Hiroshima and Nagasaki are habitable and inhabited.
Nuking Seoul would be a small price to pay.
North Korea doesn't have plans to nuke the south. They want to capture it and their industry and farms. Furthermore, the easiest way into the south in via Seoul, for all the highways to/from the south feed into Seoul. (As a result, all the bridges on the Han River and points north are built with explosives so the South can blow them up and hinder a North Korean push for Seoul.)
Nukes would destroy that logistical hub and make it much more difficult for their own military to quickly move out.
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Nukes are the deterrence to keep the US out. For they know that they can't win against both South Korea and America... but for some reason, they think they can beat South Korea if we stay out.