I don't know much about geopolitics so I could be very wrong.
The groups that would take control:
Japan: Likely already have this tech, ally of US
Korea: Likely already have this tech, ally of US
Russia: Already have this tech
China: Already have this tech
USA: Already have this tech
Or do you mean some minor group, rather than a state taking control of a failed NK?
People.
Imagine if you were given this scenario: "x" people will be killed tomorrow, you can't change that. There are three options for which people will be killed, either "well-off", "in poverty" or "random". You can choose one of the three, if you don't make a choice then one of the three will be randomly picked and the deaths still occur.
Would killing the people in poverty be better because you're wiping out more misery, would it be worse because you're letting financial state influence death?
And as much as ethically we want to think that all people are equal, let's not forget that most armys would be willing to kill 10 opponents to save the life of 1 colleague, and taking the US as an example I suspect (could be wrong..) a poll of the US would see most people willing to kill 10 North Koreans to save the life of 1 American, without having any other information about the 11 people who might die.
It depends WHY it falls. The existing system is almost completely military with a sorta civilian mostly hereditary aristocracy leading it. So if it falls because the military collapses it'll be nearly a wasteland, total anarchy, but if the military stays in total power then instead of hereditary civilian aristocracy they'll have a genuine combat soldier (or at least a semi-experienced general) in charge.
At the very tippy top the Chinese like their buffer states, and are not going to be happy with reunification with SK. Of course the same people said the same things about Eastern Germany.
The problem China does not want to have is a US ally at his border.
In the last decade or two, I doubt China wants a more dangerous North Korea. It could be that NK is attempting to play China like it plays the rest of the world. We always assume China is pulling all the strings but maybe NK knows some strings it can pull too