What makes him an asshole? He was in a really difficult position. He played lots of mind games with US, South Korea and rest of the world because he had to. This whole thing predates his leadership.
What makes him an asshole? Are you serious? Lets see... making sure his army was fed before anyone else while watching over a million (!) civilians starve to death. Forced labor camps with conditions not much better than Nazi concentration camps. Clandestine kidnapping of Japanese nationals. Stunts like the unprovoked sinking of his neighbor's vessels, and firing medium range missiles over the border, then stamping his feet like a spoiled brat with demands for increased food aid. Shooting of tourists for wandering away from their "resort" compound. Aggressively pursuing nuclear weapons and military might while ensuring that his people were too hungry and scared to revolt. Establishing a god-king cult of personality. Having anyone who wants to leave that hellhole of a country executed on the spot. Punishing entire families (including small children) for "crimes" of individuals. Secretly compiling a vast store of luxury goods and spirits for disbursement to his party elite, while claiming to be a shining example of humble socialist perfection. Raising his son to believe he's the rightful heir to the universe, thereby ensuring that the cycle of arrogance and brutality continues for another generation. That, my dear reader, fulfills my definition of a grade A asshole.
"Gotcha, you snot-necked weenies!" -- Post Bros. Comics