A few things I think some people don't understand.
1.) Seoul is a 40 minute drive from the North Korean border. We're not even talking like 3 hours apart (driving distance) here. North Korea has conventional weapons that would level the city within minutes.
2.) Even for the "Dumb American" trope person who doesn't care about Korean lives lost ... you do realize there are like 50,000 Americans in in the area right, most living in Seoul. The death toll for American civilians in such an attack could likely be more than 9/11 and Pearl Harbor combined. Think about that for a second.
3.) Seoul is not just "kind of big city". It's the 3rd or 4th largest city in the world population wise, it's basically like New York City. Imagine trying to evacuate a city the size of New York in 20-30 minutes.
I don't really see a military solution here, IMO the world community is probably going to have to accept North Korea and offer them incentives to stop being hostile.
People were so quick to snap at Obama for the Iran-Nuclear deal, but it seems to me like looking at it now that deal was a very, very good one as it prevented a situation as we have in NK now before it got out of control by incentivizing Iran peacefully to cease their nuclear ambitions in exchange for strong economic incentives.
Yelling at North Korea and trying to intimidate them doesn't work IMO, the only message that sends to them is that they need to accelerate their nuclear plans because they need that to protect the regime.