And more generally this seems to be an instance of the anchoring and adjustment heuristic. In this case the anchor is the present situation (North Korea bombing stuff), and South Korea is evaluating the acceptableness of a policy option. Change the anchor and - voila - the evaluation changes.
Taking advantage of an enemy's thinking flaws is one of the most effective ways a small organization can influence a larger opponent. Distractions and disruptions can prompt overreaction or under-reaction. In the case of 9/11 or WikiLeaks, possibly overreaction by the US government. In the case of North Korea, possibly under-reaction by South Korea.
I suspect that the recent attacks weren't planned, and were instead the result of some guy manning an artillery piece (or some guy in command of a guy manning an artillery piece) panicking and doing something stupid.
During the Cuban Missile Crisis, a US reconnaissance plane over Cuba was shot down by a Soviet missile without authorization from Moscow. This "stray" shot very nearly caused nuclear war. (For more examples of a lack of government control in the Cuban Missile Crisis see section VI on this outline. By the way, it would be interesting to analyze the plentiful existential risk irrationality during this Crisis. The Crisis tapes are now declassified.)
If the US and the USSR had trouble controlling their guns, it's likely the amateurish, heavily-armed North Korean state also does.
Let me mention Graham Allison and Philip Zelikow's excellent book Essence of Decision: Explaining the Cuban Missile Crisis (2nd ed. 1999) here. The thesis of the book is that it's often a bad idea to try to understand the actions of countries by treating them as rational actors, and it illustrates it by three takes on the Cuban missile crisis.
In the first chapter it uses the rational actor model and asks about each event "what was the Soviet Union trying to do here". It turns out to be extremely puzzling: for instance they shot down a recognizance plane in the last few days when they seemingly could not gain anything by such a provocation, they were extremely inconsistent with their camoflage (did they want to be discovered?), and they had weird reloading arrangements (seemingly signalling that they intended the missiles to be used for a first strike).
Then in the next two chapters, the book dissolves these mysteries by instead considering the actions as flowing from different institutions or individual decision-makers within the Soviet Union. For instance, two different organizations were involved in managing camouflage, and the reloading arrangements were standard operating procedure for the Rocket Forces (and made sense for continental sites). And the U2 shoot-down? Well, the air defense forces had standing orders to shoot down American planes, but they were not able to get their radar system up and running until the last day of the crisis...
That would have been a likely scenario during the Cold War, when people on both sides were really afraid of a nuclear attack. But neither a ship, nor an island, looks like an incoming nuclear attack; nor is South Korea supposed to have nuclear weapons AFAIK; and if the North Korean soldiers have any sense they would be much more afraid of their own superiors than of South Korea. And in a dictatorship where you need permission for just about everything you do, I doubt that a low-ranking military commander could sink a ship, or start an artillery barrage on another country, on his (her? any women in the N Korean military?) own initiative.
If they have standing orders like "If you're shot at, shoot back" then there's still potential for trouble. I think that, in the recent incident, South Korea was holding military exercises in or around places that North Korea claims are part of its territory (claims which the rest of the world doesn't recognize), and the exercises included things like shooting at targets. So it doesn't seem that weird for some North Korean guy to go "OMG, they're shooting at us!" when the South Korea military is nearby and shooting.
I don't think North Korea has the same relationship with South Korea or the rest of the world in general that different sides on some issue -- like gun control or abortion -- have with each other. Political opponents are engaged in a constant debate, each with the goal of convincing and winning over undecided voters that their view is best for all. North Korea has mostly cut itself off from any normal communication with all the rest of the world. North Korea doesn't seem to be interested in winning over converts or sympathizers, unlike the Soviet and Chinese communists.
I've seen the suggestion that some or all of North Korea's unpredictable provocations have as much to do with their internal politics as with any coherent foreign policy strategy.
I've seen the suggestion that some or all of North Korea's unpredictable provocations have as much to do with their internal politics as with any coherent foreign policy strategy.
It is a classic ploy for uncertain and troubled regimes to have an outside enemy to blame for problems.
My theory on why North Korea has stepped up its provocation of South Korea since their nuclear missle tests is that they see this as a tug-of-war.
Suppose that North Korea wants to keep its nuclear weapons program. If they hadn't sunk a ship and bombed a city, world leaders would currently be pressuring North Korea to stop making nuclear weapons. Instead, they're pressuring North Korea to stop doing something (make provocative attacks) that North Korea doesn't really want to do anyway. And when North Korea (temporarily) stops attacking South Korea, everybody can go home and say they "did something about North Korea". And North Korea can keep on making nukes.