Apparently that's about 10 million people in the city, 26 million in the surrounding area. Are you cool with that?
How?
I mean yea it makes sense for North Korea to pick the capital city of their closest enemy as a target but how would North Korea destroy the "entirety" of Seoul? The south has people guarding the boarder, and any missiles are going to be noticed long before they hit their target (if they even do, North Korea hasn't even shown they can fire a missile far enough to hit Seoul). At best they will cause some serious damage, but no way will they entirely destroy Seoul.