NK was already a MAD scenario before they had nukes because of Seoul's location relative to DMZ.
Is that really true though? The desire to avoid another Korean War was less about civilua casualties but rather a fear it would involve rival superpowers getting into a war with one another. The US, China and SK also fear the collapse of the NK state in any conflict that would likely trigger the greatest humanitarian disaster since WWII, with literally millions flooding acrosd the Yalu to China. South Korea has also stated it's in no mood to destroy its massive economy trying to reunify the peninsula under its rule.