Kim Jong Un isn't belligerent. The north koreans have a different playbook for foreign policy than you might be used to but its a playbook nonetheless. For their people the program is akin to something like the Apollo program in terms of national pride. Its also a dead man's switch effectively. The ruling family obviously wants to maintain their life of idyllic luxury and nuclear weapons and belligerent public addresses are a good way to make people second guess just steamrolling you over. In effect they are just playing a hand thats already dealt to continue their positioning.