Imagine that person believes, that if that's not done, alternative is WW3. Can you guarantee that's not going to happen?
What if they actually nuke something, and then China will follow?
Even if we just remove the missile sites, and any sort of nuclear ordinance on the first attack, they don't really have nearly as much as they claim capable of hitting Seoul[1], and even then it can only hit the northern 3rd of the city, which is less populated. The city itself has put a lot of work into infrastructure in preparation, enough bunkers and shelters to house 20 million. Yes, it will get hit, people will die, but as long as no nuke comes down, I'd consider than a win, for eliminating such a dangerous tyrant.
Of course, coming to terms with China will be an entirely different matter.
I don't think you have any idea how many pieces of artillery they have, nor how many tunnels there are going beneath the DMZ. They are ready to cause as much destruction and chaos as their 1950s era arsenal can inflict.
It would take thousands of cruise missiles to even dent their defenses. You can't use bombers until the anti-aircraft systems are down. It's going to take hours to finish the job, and even then who knows what they've launched in that time. Seoul is an easy target, very hard to miss.
I read(1) that only the northern part of Seoul the city is in striking distance of their (somewhat limited in numbers) long range artillery, and that there are lots of shelters e.g. the way the subway system doubles up as being bunkers.
However the civilians part between Seoul and the DMZ, home to millions, is not going to fare exceedingly well - if the North decides to "waste" their ammo on civilians instead of the military that will be hitting them back.
Indeed, it is hard to overstate the damage NK could wreak on Seoul, even in the opening hours of a full scale conflict. Roughly half of the population of SK lives in the greater Seoul metro area. Artillery barages could claim millions of lives, and wreak economic devastation, and we're not even talking about nukes yet. I don't relish the thought of Kim, backed into a corner, fearing total oblivion, his fingers fluttering inches away from the big red button.
I suspect that the US has had the capability to disable ICBMs for some time now. (See patriot missile system and iron dome and remember Regan's star wars project and its not a big leap to make) I think it's just a matter that we don't want to let would be nuclear assailants know until they have already committed.