Assuming NK is rational, I don't think they'll carry through. To me, it looks like they're just raising the ante, trying to get some leverage somehow.
For one, it won't nuke SK unless NK is on its death bed, in which case they'll lob a bomb at SK just out of spite. Reason: Seasonal winds ensure NK will be irradiated come summer, as there's a seasonal southeasterly wind that blows from the Sea of Japan/East Sea to the penninsula.
Two, China strongly recommnded they don't do it, and China has 70% of NK's imports in its hand, and also provides most of NK's oil.
Three, unless NK is really dying every day at a palpable, irreversible pace, aggressive actions will only cause everyone to punch NK up giddily. Especially if China doesn't help, or only reluctantly helps.
On the other hand, NK has a largish upper hand here, in where Seoul (1/5 of SK's total population) is within artillery range of NK, as one internet goer found out through investigating Google Maps. Seoul's practically blanketed with artillery fire ranges. Because SK tends to think about minimializing citizen casualties, that's a huge thing to consider. Also why the US and SK haven't covertly bulked up troops and waged a three-day war—Seoul is basically hostage here. Roughly 10,000,000 people.
The weird thing is that this is a really conditional upper hand. Once NK uses it, NK won't have a chance against the many layered alliances and SK's immediate response. Outdated soldiers can only go so far. Though SKorea will most likely be half dead by then.
So, NK has a card that is influential but a losing card, a newly found card that's all-powerful but hard to use, and ... pretty much it. I think. O_o