Perhaps I misremembering my younger years but I don't remember Kim being so brazen as to assassinate someone outside his country and then make demands of said country.
Didn't he famously had a couple of South Korean directors abducted to bolster the NK standing in film, among others ? Kidnapping random (not even persons of interest) foreign citizens on their soil with any regularity is pretty wild.
Also with non proliferation as the supposed international consensus, the international community cannot exactly just shrug at the DPRK becoming a nuclear power.
They already are a nuclear power. Estimates of a minimum of 6 capable nukes to upwards of 60. They just lack accurate long-range capabilities to, for instance, reach all of the US.
We are already in a defacto nuclear deterrent relationship.
I don't think it changes my point whether they're technically on the verge to / already a nascent nuclear power. The international community is forced to have a harsh stance on the issue. Ultimately you're right it will be accepted de facto but for now they have to go through the motions of preventing it. Otherwise we might just tear the NPT... Though there's an argument we should already because it's unsustainable.
The other problem with NK, as far as I can tell, is that it is unclear they really want to sit and negociate. I'm not sure it's Western bias to say they have a pattern, more so than most other countries (say Iran), of barely giving anything in return and outrageous demands on top of the fact there's not a lot of pressure points to apply. Even most academics seems to agree China doesn't have that much clout over the Kim's regime. Without being hawkish, we may be at a point where most of the diplomatic options are depleted and a credible threat of overwhelming force has to hang in the background. (Which contradicts my earlier message but heh... Not that I wish for it, esp. with Trump conducting.)