North Korea poses the most risk to South Korea. Any efforts to stop Kim Jong Un must include and be headed by South Korea. To maintain stability in the region, it is in the best interest of South Korea to take the lead in any efforts to oust Kim Jong Un.
South Korea should definately help the citizens of North Korea and try to stop Kim Jong Un and the rest of the people who are keeping them in power. But in order for them to be able to do so they need the help of the other UN member nations.
No, South Korea shouldn't intervene with Kim Jong Un since it would be placing its own people in harms way. Kim Jong Un should be taken care of by different major nations at the same time to minimize any damage that would incur to the neighbouring countries. At this time, South Korea doesn't have the proper means to do this itself so it should hire the aid of the world super powers to take him down.
No, North Korea has nuclear weapons and any provocation by South Korea could lead to the first nuclear attack since World War II. By attacking North Korea, South Korea would draw its allies into the conflict, including the United States. While stopping Kim Jong Un's oppressive regime is important. The cost of a nuclear detonation is too high a price to pay.