I think it might be legal to hide the body, but if you do so you must do something to ensure that it can be recovered, either informing the enemy afterwards or some other measure to that effect.
If IHL were deeply ingrained in international political behaviour and absolutely established I could see deviations from it where such deviations could be justified on moral grounds as acceptable, but since the law is hardly even established it is more important that it be followed than that it minimizes suffering in the now.
Furthermore, I don't think that it mattered. Everyone places his tactical concerns very highly in the moment, but over time they are often irrelevant. Do you really think it matters now, whether that information was available or not?
I don't really believe there's anything useful that can be achieved when it comes to the North Korea's nuclear weapons. They have them, they'll probably try to build more of them. That sucks, but there's nothing that can be done.
I don't think we should be having wars and killing each other. I don't think we should need to. Hopefully one day we won't and we will stand together as 1 species united in purpose and prowess and exploring together.
For NK, projecting strength is so important. But belligerence is all they have, they don't really want to be fighting, they want to be rich and having fun. Past humiliation and present partnerships coerce them towards war. Surrender is improbable, and for them joining the world can only be done on terms where everyone respects their strength. Tho maybe it will change, and they will hunger for what we have, more than they are afraid to stand as equals lest they be seen as weak.
I think what can be done is to bring everyone together so no country needs nuclear weapons pointed at another country. But until then you need advantage in case of hostility and conflict.
If you could stop NK doing that, would you pull the trigger? Would you make a targeted kill of a person who compromised that mission by discovering it?