First note that I said the it is not impossible to treat different countries differently. I will also grant you that North Korea is objectively a worse place than the USA for the people living there. But we are debating whether North Korea should be allowed to have nuclear weapons, not how they run their country or treat their people. If the USA can justify their nuclear weapons with foreign threads, then North Korea can certainly do the same.
If we still conclude they should not or must not have nuclear weapons, and I tend to agree with that, then we have essentially three options. We can nicely ask them to stop and optionally offer some kind of reward or compensation if they do what we want. If that does not work, we can impose sanctions on them and we can do so without any additional justification [1], if we don't want to trade with them because they have nuclear weapons, that is our decision alone, it does not really matter what our reasons are. Obviously some objective justification would make us look much better.
If that still doesn't work we can either either give up and let them have nuclear weapons or we can use force against them, whether sabotaging centrifuges or bombing reactors. But now we need a solid justification for our actions unless we are willing to accept that the same actions against us would be equally justified. Also note that we are just eroding the sovereignty of countries.
We could treat the world as a kind of democracy, if most countries [2] agree that a specific thing is not okay, then we are legitimized to enforce this with all means necessary. We may or may not be able to objectively argue that nuclear weapons in the hands of North Korea are an exceptional danger but that doesn't really matter because our legitimization comes from the majority vote and not from any good reasons. It would of course again make us look much better if we could come up with some reasonable objective justifications for why some countries may and others may not have nuclear weapons.
There are of course also implications here, for example if the majority of the countries decided to ban nuclear weapons altogether [3], then any state including the USA would have to give up their nuclear weapons.
[1] There are of course complications, we may, for example, very well punish innocent citizens for the actions of a few at the top of the state.
[2] Maybe weighted by population or something along that line.
[3] https://en.wikipedia.org/wiki/Treaty_on_the_Prohibition_of_N...