Sure, Donald Trump is so callous and crass that when he says stuff, it sounds awful BUT extended deterrence (or even regular deterrence) almost always threatens killing lots of civilians. The whole idea is that if you do x, we will make you pay. And the "you" in this is not just the leader of country A but all of country A. Mutual assured destruction? Kind of sounds like that destroy word Trump used. So, the speech is not really a break in policy, as awful as it sounded.
Ethicists, moral philosophers and others have long consistently raised questions about the morality of nuclear deterrence because it requires holding innocents hostage. I am not an ethicist or a philospher. Instead, I focus on what seems to be the least bad alternative, and deterrence when it is successful is better than war. So, I don't have a problem with Trump saying that the US will use heaps of force if North Korea attacks Japan or South Korea--the whole idea of the "nuclear umbrella" is that the US is potentially threatening utter destruction if someone attacks a country who is under the umbrella. Who are those allies? NATO + Japan + South Korea + Australia + New Zealand (maybe, the whole nuke free thing made that less clear). That's it. Baltics? Yes. Sweden? No. Mexico? Hmmm... not quite although that kind of falls under the whole Monroe Doctrine of don't mess around with our neighborhood or else.
What is more disturbing about Trump and North Korea is that the US has made the other half of that promise problematic and Trump makes it even more so: that the US will not attack you if you don't attack its allies. Qaadafi might have something to say about that .... Talk of North Korean regime change is DANGEROUS because they might feel they are in a "use them or lose them" situation if they think the US is about to disarm North Korea. Which gets to the McMaster McMess du month: saying that there are military options is reaaaaalllllllllly problematic when North Korea can kill tens of thousands of South Koreans under the best scenario and millions of South Koreans and Japanese and even Americans in the US under the worst scenario. I would be assuring the North Koreans that the US is not going to attack unless North Korea attacks our allies. But Trump ain't doing that, the US has a poor record on that score, and Trump cannot make a credible commitment--it is not in his nature.
Alas, deterring North Korea from developing nuclear weapons has proven to be a failure. But deterring the development of nuclear weapons and deterring their use are two different things. And, yes, breaking the Iran deal is a poor signal to North Korea if we want to deal with them.
So, yeah, I am worried. I spent this afternoon giving a talk to a group of Asia-Pacific Ambassadors based in Ottawa. I think I depressed them. Damn.