[One problem could be her perceived need to demonstrate toughness in that culture.]
A dictator has to ward off potential challengers with exemplary killings - oppose him and you and your family will be killed as an example to all who might make a grab for the throne. While our caricature of dictatorships is of drones in thrall to the queen of the lot, in reality, they are nests of intrigue, with any number of individuals and cliques all looking to put themselves on the throne. So if she’s next in line, she needs to kill potential rivals in order to consolidate her power. If she is wrong-footed, she will be killed, probably with the rest of Kim Il-sung’s progeny. If she goes down, what’s equally possible is a puppet, perhaps a child, from Kim Il-sung’s bloodline being put up as a figurehead, while the true power rules as his guardian. Not much different from Japan’s thousand-year rule by shoguns in which emperors were puppets on a string.