North Korea doesn't seem to be aware, but not only they might kill millions of people, but also North Korea can get wiped out, that is millions of North Koreans can also die.

That means that both North and South Korea can pretty much get "wiped out", that is, no more Korea at all,  that also means, no more North and South, just death every where on Korea, that would mean that Korea could start as being one country.

The cost? Maybe millions of people dead and also billions of dollars wasted.

North Korea is not aware that they can all die and they can take the South Koreans with them.

Also, China is not aware that it is very much possible that a few lost nukes from the war might land somewhere on China and kill a few thousands or millions of Chinese, as to Japan, who know, but they might also suffer, if nothing else there might be massive radiation doses and nuclear waste everywhere, on land, air and sea.

How long would it take for the nuclear waste and radiation to go away?

They probably need to drop a few giant non nuclear bombs on North Korea on their capital and also wherever they have their weapons and military bases.

They need to do it before the North Koreans kill everybody on South Korea and or somewhere else.

Who really wins? The only one who wins always is good old "Beautiful Light" "The Morning Star" "Luzbel" the angel of light, Satan (Satanas) or Lucifer,

The father of all lies.