I mean, let's look at the third-world dictatorships widely condemned for their WMD programs. Gaddafi and Saddam ended their WMD programs and were overthrown due to U.S./western military action. Iran negotiated with us, kept their end of the deal, and we broke a treaty we'd made with them less than a year ago for literally no reason. In other words, we proved that America can no longer be trusted to honor it's treaty obligations even if the other country does everything it agreed to do. Now let's look at Assad who not only refused to abandon his WMD program, but gassed his own people after the U.S. basically threatened him with military action if he did so. Assad is still going strong and now has a superpower (Russia) backing him while the U.S. has simply washed its hands of the whole conflict.
If you're a rational actor in Kim Jong-Un's position, the logical thing for you to do is to refuse to even negotiate about your WMD program and bet that you can either keep the game of chicken going until Trump loses in 2020 or simply scare Americans badly enough with your saber-rattling that war of any sort with North Korea is politically unfeasible for Trump's administration. Honestly, I think Kim Jong-Un is far more of a rational actor than Trump. Kelly, Mattis, etc are rational actors, but Trump clearly isn't.