Anti-missile systems aren't really designed to deal with artillery, plus regardless of the type, quantity and quality of any defensive system the main problem is that the short distance would result in a nearly non existent warning time. If North Korea actually attacks someone retaliation from South Korea and America is 100% guaranteed, Pyongyang (and with it the NK regime) would be a smoking pile of rubble within hours. The problem is that until then Kim Jong-un as the head of one of the worlds largest armies would easily have the opportunity to go down in the history books as a monster who killed millions as the world would see it, or as the greatest martyr who ever martyred (if he believes his own propaganda). NK would certainly try to stop people from leaving, but the general expectation is that that would cease to apply very quickly as loyalty based on starvation and fear is unlikely to last long if the regime fell. China wouldn't really be able stop them from coming, what would they do? Shoot unarmed civilians? All of the above is part of the reasons why so far no one has actually taken any real action against the NK regime and why the NK regime gets off on posturing rather than shooting first.