My first inclination is to tell you that this wasn’t ‘technically’ a US-NK war. IIRC this was a United Nations ‘police action’ that was under the aegis of the United Nations.
So, given that if we go in and assassinate him and admit it, then our defense is to rely on the UNITED NATIONS to now say “yes, we were at war,” supporting us. Really? You believe that group of bastards would back us up?
Trump does not need or care about UN approval.
Besides, there really is no “law” that forbids killing Kim Jong Un