None of this is remotely recent news and it's assuredly horrific, but why are you so eager to doom millions to death based on a hunch and an incredibly over-simplified understanding of world history?
Evil is not so unpredictable as you suggest. It is not innately irrational. No individual sets out with the goal of destroying themselves. The logic of tyrants can be studied, understood from the outside, and reacted to. There are experts who have studied North Korea for most of their lives, and they don't believe we should strike at them. The essential flaw with your Nazi comparison is that the Nazis attacked other countries for what they perceived as the betterment of their own people. North Korean leadership is unconcerned with the well being of its own citizens and is exploiting them instead of surrounding nations, because expansion is (and always will be) impossible.
What the North Korean leadership is doing to their own people is barbaric, but it's no reason to sacrifice all of South Korea.
Again, I never meant to equate Nazis and North Korea. I used them (specifically the way they acted after Stalingrad) as an example of a group of people not acting in their own best interest, even disregarding the primal instinct of self preservation.
Of course dictators follow certain principles, but not always. First example that comes to mind: Idi Amin. We really don't know enough about Kim Jong-un's state of mind to allow any such presumptions.
What has been happening in North Korea for decades is a humanitarian catastrophe. In an ideal world, the leaders of the world would have come together and ended it long ago. Now that NK has gained nuclear capability, it has turned from a local problem into a global one.