What's the worst thing that can happen?
Seoul has about 25 million people. Most live in 50 story apartment buildings. North Korea has between 10,000 and 20,000 artillery pieces aimed at Seoul.
I'm unsure how many artillery strikes before an apartment building collapses, but picture several thousand World Trade Center collapses all at once, each one with thousands of people trapped in the rubble.
More importantly, do you think you might have reasonable knowledge about KJU's nuclear capabilities?
North Korea can fire atomic warheads at Tokyo and Guam.
They cannot reach Hawaii/Alaska or mainland US.