They have enough artillery pointed at Seoul to cause mass civilian casualties. It would be a humanitarian disaster on both sides of the border. Now, in a case

They have enough artillery pointed at Seoul to cause mass civilian casualties. It would be a humanitarian disaster on both sides of the border. Now, in a case where a (conventional) war broke out between NK and SK without China's involvement, that artillery would be carpet bombed to oblivion by US air superiority in a matter of hours/days. But if the US was tied up elsewhere, or China was providing support. Nightmare scenario.