His son was reportedly behind the shelling of South Korea a while back. Reports vary, from batshit crazy, to a bit more levelheaded than his father. However, with the dearth of information from the inside power structure of the North Korean government there's no way of knowing.
Know this: the centralisation of North Korea's government just blew wide open. There's a good possibility of a general to initiate a coup; a way for his son to assert power might be to take control of the military immediately and do something drastic, including the attack on South Korea. South Korea's capital, Seoul, is within shelling range of the DMZ, and especially with their military power, North Korea can seriously do some damage, costing many lives.
China also comes into play here; it is in their best interest that North Korea doesn't start starting wars. Chinese diplomats have probably been dispatched immediately to grasp control of the situation. Furthermore, the eldest brother is in custody in China; in order to install a more "China-friendly" government, it's possible that they may try to intercede and put the eldest in the position of power.
On the subject of reunification, North Korea's GDP is about 20x lower than South Korea's; the damage it'll do to South Korea's economy will be disastrous if there's a reunification. The best thing would be to slowly open the borders for humanitarian aid to go in, and for North Korea's economy to build itself up. It'll take about a generation or two for a complete reunification in my opinion, but this is the step South Korea's going to have to take in order to not completely screw over its denizens during the reunification process (although more hardliners will push for a quicker reunification).
@some hero: Are you batshit crazy? You want the US to go in without reservation and start another war to boot in Asia, while managing to completely fuck over millions of people in North Korea trapped in an oppressive regime? Not only will this garner more ill will (heh as if America can get an even worse reputation), it'll be a humanitarian disaster.
I'd also like to extend a huge FUCK YOU to some hero, since clearly allowing China to take over an ethnic group shared by the South is the best idea. Do you honestly fucking think China would want to give back North Korea after annexation, or it's about good will?
Edit: Jesus, reading through this thread, it's clear that some have no fucking clue what they're talking about.