At this point the moral thing to do might actually be to invade NK and save their populace from the "government" they currently have.
The oppressed cry out for democracy! And the U.S and her fellow capitalist shall deliver it! As soon as we move our drones from the middle east to asia, that is.
Oh, come on. I think it is a little silly to pretend that there is not a moral imperative in this situation. This is North Korea we're talking about here. The ones that run actual, honest-to-god concentration camps? That won't even help their own people when given the opportunity?
There are a lot of morally grey situations in politics, but this ain't one of them.
The issue is that a distant foreign power trying to unilaterally do so creates a ridiculous amounts of issues. If something like this was done, S.Korea needs to lead it, or alternatively, the UN. Even China might be a better option, since they can soak the negative PR much better than the US, and still have the excuse of a "concerned local power".
That said, if N.Korea shoots first, well, no one reasonable is going to blame the US when they go in, depose their regime and impose a democracy. Except China, but that's just because they don't want an American puppet on their borders, not out of any love to N.Korea. In fact, they might decide to invade N.Korea and depose their regime before the US does it, just to keep the new regimes at
their terms, or at least have some grounds to find a better (to them) compromise.