It's actually quite hard to defend a city from ballistic missiles. We really can't do it all that well - the American Patriot system is the only one that's actually tried to do so in 'live' situation (one of the gulf wars) and opinions of it's success rate is varied. Moscow in theory has ICBM defences but obviously they;'ve never actuallybeen used, so nobody knows how effective they'd. NK almost certainly doesn't have the types of high speed electronics, missile tech and radar systems to build something like a Patriot. If Seoul has enough long range missile artillery with big MIRV incendiary warheads, they could set a fairly big portion of a city alight and then it's a case of whether PyongYang's civil defence people are up to fighting widespread fires simultaneously or not. You don't actually have to set fire to all of a city, just enough so the locals can't put all the fires out, then let nature take it's course.
Not that they'd actually ever do this in practice - it's definitely falling under the category of 'war crime', and the nations who were otherwise on their side would drop them like a hot rock if they did it. They may be able to decimate Pyongyang, but they certainly don't have the conventional armed forces on their own to stop North Korea coming over the border afterwards and stomping them into the ground.
