Kim Jong Un is batshit crazy, but he does know the extent of his power. His power doesn't reach much further than North Korea itself, and if he tries anything that might piss off China, he backs off. Trump has no such restrictions, he can do whatever the hell he wants, and does.
The general principle in international relations for the past 100 years is that if you kill your own people, nobody really gives a shit. If you start killing other people, well then that's when other countries start taking notice.
He doesn't seem to have much of a plan. He must be dying very fucking fast or something because it looks like he's just trying to cement a legacy as quickly as possible before he pops his clogs.
The plan appears to be:
Rinse repeat until the country collapses. No attempt at stabilisation, no positioning of the opposition so they can take over, just bomb bomb bomb and we'll think about what to do afterwards when Trump is dead and doesn't have to think about it anymore.