Seems to me the answer to this must be qualified by the reminder that like many other characters, the NK is something conjured up by the author to complement part of this story line and cannot be analyzed in terms of anything real.
I do not mean to insult anyone by saying this. I know you understand this. So why am I repeating it?
Well, so many posts made here seem to be based on the fact the chars are somehow real or at least can be analyzed according to human psychology. But we all know they are not and yet, that just seems so easy to forget when discussing them - especially chars like NK because it's not even based on anyone or anything real.
My reply would be the author invented NK because he needed some symbol of fear and terror to justify the development of the other chars. If the NK was real, I think the way it's been constructed just omits far too many human characteristics. Specifically, I don't think it has ever shown any sign of the ability to make complex plans or to think of ways to out-maneuver opponents. Apparently, that part of its mind died when the human part of this char died.
There's almost nothing about NK that makes any sense to me. It wants "death" to prevail and all living things to die so that it can then rule all that remains. But everything that remains will be dead. How does that make any sense?
It seems clear to me that a great truth has been made in this show. I refer to the truth, "nothing ever stays the same". So the condition where everything on this planet is dead and NK rules over all of it is impossible. Why? Because that would be a situation where nothing ever changes and there would be no point to that. There would be nothing to gain and nothing to lose. It makes no sense. It was never intended to make any sense. It is just an empty figment used to help make some sense of this story line and - IMHO - it is very poorly constructed. It's just one more external threat designed to set up yet another big expensive battle scene.
Personally, I'm tired of all these big battle scenes. I know they cost a whole lot of money to produce and they are quite thrilling and make for stunning TV. But when you watch battle after battle, it doesn't take long before they become excrutiatingly tiresome. How many battles can you watch before you question the point? I understand that someone always emerges from these things as extremely courageous and a great hero. But this cast is just chock full of extremely courageous heroes. There comes a time when it all just becomes too tiresome for words.
I truly do not mean to criticize anyone for believing it's possible to use human psychology to analyze chars created from the author's imagination. But after seven seasons, I just can't understand how the show runners expect to maintain the high quality of this entertainment.
Like many others, I believe this show is quite possibly the greatest TV epic ever made and I've loved watching it. But, it's just becomes far too difficult for me to go along with it anymore - at least the parts of it like the battles.
I understand that I can't speak for everyone. Many of you may have no problems to accept what is given to the audience. Perhaps I'm just getting too old for it any more.