It will be a while before North Korea's missile technology can reach the US (in term of range). So far all they do is blowing up poor fish left and right every time they do a test. Even if they successfully developed such technology, it is not guaranteed they will be able to strike the US mainland. Their productive level will not allow them to mass produce a lot of such missiles even if they are given such technology. It is a long way from North Korea to the US mainland, the limited number of missiles can be easily intercepted and any kind of retaliation against the small North Korea will severally damage the North Korea.
One thing you can be sure, no matter how crazy the media make the Kim family look like, their priority is to hold onto their dynasty power in Korea, and maybe their final goal is to reunite the two Korea. The nukes they develop is purely for scaring off any possible invasion against them and anyone who dare to intervene. If they goal is actively seeking to destroy any country, then their own days are numbered too. Why would you want to commit MAD while you can rule over 23 million poor people and stay fat and powerful in front of them?