No-one wants to invade DPRK. Not even South Korea. For starters, ROK's king is in the palms of NK—Seoul, where I happen to live, houses 1/4 of Korea's population, and is only a short car drive to the DMZ. Artillery works quite well against totally unfortified civilian buildings that were built without a thought for the northern menace. Also, if there's enough targets to shoot at you'll kill a lot just by trying.
Second, even if ROK gets NK in a totaly peaceful and agreed-to matter (let's say Kim signed over the country to ROK... somehow), the rehab of the two nations' citizens is going to cost a lot. DPRK has nothing to offer ROK right now. Consider that Germany was richest of the European nations when they reunited, and the wealth difference between the two sides was only a few times. They're struggling too, I think.
DPRK and ROK have a wealth difference of fifty times or more, and ROK really isn't that rich compared to other nations. The cultural difference is outstanding, too, different dialects, different vocabulary, different cadence; all will mark the DPRK citizens as to who they are and serve as an excellent target for discrimination. ROK is homogenous, like Japan, and doesn't tolerate 'different' people well. It's easier if you're totally a foreigner. Mixe bloods between, say, Vietnam and Koreans are harshly discriminated against. Now, think of DPRK citizens, who look the same, but are radically different in thinking and speaking, and are probably poor, and already have a bias of being loony cultists. They are undertrained, meaning they'll tend to be in the lower jobs, and the bad image will be confirmed.
Third, think of China. China likes having DPRK as its vassal, who woul leap if China say leap. DPRK is a nice buffer, and guards the land route along with serving as a good threat to maneuver in politics with.
Next, remember that DPRK is a nation backed into a corner. It's allies disappeared one by one, climaxing with the USSR collapsing and China embracing some aspects of capitalism. A rat surely will hurt the cat threatening it, and maybe kill the mouse that it knows is an inside man and within paw's reach. He has a nuke too, that can harm a good chunk of nations. The burgeoning economic power of ROK makes DPRK's propangda next to useless, so it doesn't have non-violent weapons left. The culture of ROK is infecting DPRK, smuggled and traded via the blackmarket.
Cutting off aid will worsen the humanitary conditions.
Yeah, I'm rather pessimistic about us being able to do anything decisive right now. Only time will show us a way ... >.>
/[end uncited rant]