It is probably unlikely that N Korea has the ability to hit Guam just now. Hence probably the reason why they announced in such elaborate detail their plan to attack. Pure bravado.
I wouldn't want to say that the N Korean leadership is unstable though. They know what they are doing. The people in real danger here are the S Koreans: that is probably the insurance Kim has that his threats will not be answered with real "fire and fury". They are very much in range of conventional weaponry. The problem will arise if and when N Korea really does develop its nuclear capability to the full. Then they may well be untouchable, either as a guarantee of their survival, or even, as some have suggested, as a way of attacking the south conventionally with the nuuclear option as a deterrent to anyone stopping them.
That is why really diplomacy is the only way to stop this escalating.