You may be giving more credit to N.K. than is due... I view the elder leader as a very dangerous individual who does not use logic to make decisions.
If I were to blindly read the stories (not insinuating you do), then his brash actions may indicate a level of insanity a little above normal.
However, his country's citizens are not pulling a coup d'etat, which means a) they are satisfied with the life they are living, or b) he effectively keeps the in check with fear; maybe a combination of both, after such a long time in the leadership position.
However, his coyness in playing ball only for a good few months, then changing course indicates it really was a game of diplomacy or whatever you wish to term it. The best defense is a good offense, and by participating in the negotiations for some time may have allowed him to execute some infrastructure moves, which would pave the way (pardon the pun) for reigniting the original plans.
Which would indicate a level of logic not always seen by the general public (me) of their leaders (the PotUS).
So, while you may not think North Korea's leader uses logic, I think his actions indicate a more hidden agenda than meets the eye (at first anyways).
For example, I remember seeing some "Battle 360" program on the History Channel, where a smaller unit of forces retreated into a smaller area, thus limiting the power of numbers of the larger army.
It is a long analogy, but I think it applies.