Kim Jong Un is terrified of an overthrow. The execution of government officials since he took power is remarkable. Anyone that he even remotely suspects isn't 100% loyal is eliminated. The assignation of his brother (who was pro-west) was likely done to eliminate a potential rival for power. It has been reported that he replaced his North Korean body guards with Russian mercanaries. But eliminating top military leaders leaves his Army with loyal, but perhaps unqualified leadership in the event war breaks out. Kim may legitimately believe that the nuclear weapons are his only protection. But he may have it backwards. They greatly increasing the likelihood of outside attack, or a coup of some sort. And he wont survive 2 seconds of either one.