I think it's dangerous to simply write-off Kim Jong Un as a "lunatic" or "crazy". There's a reason why experts on North Korea do not categorize him as such.
All three of the Kims have been ruthless, brutal, belligerent, and seemingly unpredictable to the American mind. However, there is a very cold logic that underlies the actions they take for each audience they deal with: the North Korean population, the inner circle of cadres around them, and the international community. That logic is always about self-preservation and, to a degree, maintaining the status quo.
I strongly recommend reading The Cleanest Race
by Bryan Reynolds Myers to get a better understanding of how the actions of the Kims has supported those goals of self-preservation and keeping the status quo. If not, at least skim the Wikipedia article: https://en.wikipedia.org/wiki/The_Cleanest_Race
North Koreans are not an apocalyptic people. There is no concept similar to Christianity's Judgement Day in which the wicked are removed and the righteous survive. As such, there is not a desire to engage in any kind of existential war with their enemies. The reason for developing nuclear weapons and ICBMs isn't to actually strike the United States, but to make any kind of military conflict with North Korea so unimaginably awful that no western power would dare try it. Yes, this is classic deterrence theory. Even on the more conventional front, North Korea knows that in any conflict which it initiated, China would not provide man-power assistance in the way it did in 1950, either for offensive or defensive purposes. China may be willing to risk confrontation over Taiwan, but not over what it essentially views as a tributary state.
And this gets to maintaining the status quo. From time to time, North Korea engages in some belligerent behavior - sinking a South Korean ship, lobbing mortars across the DMZ, threatening brimstone and fire (by the way, North Korea's insults are just absolutely top-notch on creativity) - and it does so to maintain the current state of affairs. North Korea, by which I mean the regime, cannot survive on its own internal resources, no matter how much it is willing to brutalize its people. If it could, it would retreat into its shell, completely seal all borders, and basically never speak to anyone. Since it can't, it has to periodically reach out. However, its ethno-nationalist ideology does not allow for handouts. It has to feel like it's taking what it needs, not being gifted things out of mercy because others pity it. The cycle of belligerence - crisis - pullback is fundamentally about ensuring a steady supply of resources that it doesn't have to pay for (since it can't).
North Korea lives perpetually on the brink of catastrophe and that's by design. The west will not allow tens of millions of innocent people to starve to death. It's fundamentally antithetical to our culture and so when that gets too close to happening, we send food and medicine, like after the famine in the 1990s. From 1995 to 2008, the United States provided $1.3 billion in humanitarian aid. 2017 saw the first shipment in 9 years, $900,000 worth, because North Korea is once again teetering on the edge of a famine. On the other side, China does not want tens of millions of impoverished North Korean refugees streaming across the border, so it sends industrial aid like coal and steel (typically paid for with North Korean slave labor, for what it's worth).
This is the fundamental balance. Pyongyang must feel that it does not face a threat to its very existence, so it builds up defenses - including the ultimate defense. It must also strike a balance between being belligerent enough that it can't be ignored so that attention is constantly focused on its delicate balancing act. It's extortion, yes. It also works.
As for taking out the Kim regime, I am conflicted. I'm appalled that a country like North Korea exists in the 21st century. I have read many books of first-person accounts from defectors (Nothing to Envy
, The Aquariums of Pyongyang
, Escape from Camp 14
, etc.) and it's just horrifying. In that sense, I want something, anything
to happen to end the suffering. But I know that there is no action that can be taken that will not result in the deaths of millions at a minimum. And so with North Korea only superficially threatening the United States, the cost of taking action is just too high. In this lose-lose situation, I opt for patience in the hopes that the suffering of 25 million North Koreans is not eclipsed by the death and suffering of tens upon tens of millions of North Koreans, South Koreans, and possibly Japanese and Chinese. Patience is the hardest game to play in realpolitik, but it's often the right play.
As to where that brings the United States, the cycle is currently unstable... not because of Pyongyang, but because of Washington and the current president. While I am increasingly concerned about his mental health (there are numerous signs of dementia setting in, including his ability to gaslight himself over the veracity of the Access Hollywood tape and other topics), Trump does fundamentally operate on a logic, as disturbed as that logic may be. In his case, everything comes down to making sure "you" lose. If it's a lose-lose, as long as you lose more than him, he still wins (see: casinos going bankrupt). That's the true threat that we face in destabilizing North Korea; not Kim Jong Un, because on this topic, the United States can always make sure it kills more North Koreans than Americans will die.
Strategic patience is the right tactic, as unsatisfying as it is. South Korea plays it perfectly. It's annoying as anything when North Korea gets all pissy with everyone, but you grin and bear it, provide enough food to keep North Korea from falling over the edge, and move on. The situation will only be resolved without millions of casualties if the change comes from within the North.