Like his father and grandfather (and Putin), Kim loves being unpredictable. There are three reasons for this. First of all, the element of surprise is obviously a strategic advantage. Second, it makes him the focus of attention, and boosts his ego. Third, the use of arbitrary power makes it clear to everyone that you are the man, and that the only sure way to avoid your wrath is to provide unconditional loyalty.
Trump values unpredictability for all of the same reasons. The difference is that Kim and Putin aren’t really accountable to anyone in their respective political systems, while Trump is the leader of what used to be called the “free world.” He has different obligations to the public. He just doesn’t acknowledge them; to him, being president of the United States is no different than being the CEO of his companies.
Is Kim bluffing with his threats to call off the summit? Most likely, he is sending a message to Trump that both of them can be capricious, and trying to find out exactly how desperate Trump is for a “win.”
The most likely outcome is still that the summit will go forward and that Trump will give away the store, but let’s be honest–this could go anywhere.