What would the ulterior motive be for taking on the cost of a war against NK? They have little to no oil or other super-valuable commodities as far as I know, so there's little financial motive. They're a risk to their neighbors but are not a credible global threat, and they're less likely to attack the U.S. than more dangerous and ideologically fervent enemies like ISIS.
They're more of an annoyance than anything else. The strategy so far seems to have been to wait it out and hope their crazy cult-regime finally collapses. I don't see any reason anyone would want to expedite this unless they thought there was a clear and present danger of NK escalating in both belligerence and power in the near future. I can imagine the former, but not the latter.
The value of NK isn't anything internally intrinsic; the main reason it has "survived" with its current and past leadership is that it is a buffer between the "West" and China. And previously it was seen as a buffer between Japan (and also the US) and Russia; Stalin was the one who authorised the Soviet invasion of Manchuria in the very last days of WWII. Russia's leadership at the time remembered Russia loss of face (and territory) to Japan in 1905; remind you of current day invasions and annexations with ruminations of nationalism?
The Korean War was in many ways similar to a "more power" version of Ukraine today.
NK may be a sideshow as far as a lot of interested parties are concerned. The benefit (if you can call it that) is that the presently-tapering security contractor gravy train gets a new shot in the arm, the Feds have a chance to set up "partnership" agreements with any number of tech firms that they just alienated during the NSA fiasco, and a much higher level of generalized network surveillance becomes a thing that plenty of companies start thinking they want.
I obviously don't know the story behind this. The point is, there are plenty of opportunistic and self-serving domestic interests who probably don't either but can nevertheless turn this hack into a very beneficial event, regardless of who did it. From their perspective, NK being the source is actually the best-case scenario.
Punishing defiance can be an ulterior motive. When the mafia send someone in to break the shopkeepers fingers they often risk their guy getting arrested, etc. losing more than the shopkeeper is worth. They still go through with it because that shop isn't the only thing in the balance, it's about keeping all the other shops scared and maintaining "credibility." You can hear war mongering politicians talk about "credibility" and "credible threat of force" all the time.
It would drain China's resources and focus, Russia's for that matter too. But, again, I said that I don't think that's what's happening with NK. NK is even too much of a pandora's box of crazy that I think will only be set off as a last ditch effort against, mainly, China. I see it as kind of a bomb that is surgically attached to China, which we can somewhat easily figure out how to set off if we so desire.
Drug running, counterfeit currency production, kidnapping foreign citizens, nuclear testing and producing nuclear capable missiles and testing them in provocative ways.
Plus their prison camp system, internal mass famines, brutal authoritarian dictatorship.
They're more of an annoyance than anything else. The strategy so far seems to have been to wait it out and hope their crazy cult-regime finally collapses. I don't see any reason anyone would want to expedite this unless they thought there was a clear and present danger of NK escalating in both belligerence and power in the near future. I can imagine the former, but not the latter.