A smart KJU would understand _that_ to mean full destruction of NK is in place and that Donaldus Maximus (henceforth, DM) is willing to do it.
It is not a threat to take out just KJU, DM knows it is illegal to assassinate a foreign leader.
Pompeo is doing a great job in Senate hearing right now. Menendez is being a real ass, but Pompeo notes that the US is ready and eager, but NK has not followed up with requested prep meetings.
Menendez shifts to Russia, and accuses Trump of being soft on Russia (for lack of rhetoric). Pompeo fundamentally disagrees with "no action against Russia" and has prepared a list. Trump's action has been stringer than Obama's, and the complained Russian action took place during Obama administration.
Pompeo is really good. FoxNews covering the hearing more than other networks.
We are still in a state of war with NK. A decapitation strike wouldn't be illegal. Would meet the LOAC (Law of Armed Conflict) principles: military necessity, distinction, proportionality. Much better that the alternative. The new question is who's head gets removed?