Since North Korean leader Kim Jong Un took power in 2012 he has systematically replaced all the old-school generals with younger men selected for loyalty to Kim Jong Un and his new strategy of dependence on nuclear weapons rather than the threadbare million-man North Korean military.
Many of the retired (or executed) generals still believed that the army was North Korea’s primary offensive weapon. They were wrong in 2012 and once Kim took power, he diverted a lot of money and resources from the conventional armed forces to the ballistic missile and nuclear weapons programs. Because of this the North Korean army has fallen far behind the modern, well trained and equipped South Korean military, which has half the personnel of the North Korean adversary. South Korea has a modern air force and navy, including F-35 stealth aircraft and a new class of “helicopter destroyers” that look like small aircraft carriers and have been modified to handle the VTOL (Vertical Take Off And Landing) F-35B. North Korea has nothing to match, much less defend against these systems. Observing that, Kim concluded that a nuclear threat was the only option North Korea could create to match the South Korean forces.
In 2012 Kim believed that too many of the old-school generals still believed that their troops could invade and conquer South Korea. Kim feared that such an effort would not only fail but would give the superior South Korean ground forces an excuse to invade and conquer the north. China would never allow that but as Kim let his ground forces deteriorate and refused Chinese advice to modernize the North Korean economy, Kim feared the Chinese would not intervene if the South Koreans advanced north. This would mean the end of the Kim dynasty and China was considering that as a useful, for China, outcome. China could still intervene to stop the South Koreans and replace the Kim government with one more responsive to Chinese advice.
Kim also feared that the 2012 generals might conspire against him, in cooperation with the Chinese. It would not be the first time that paranoia shaped North Korean policy.