As I understand it, Mr. Kim actually inherited his position from Dad. That means he's likely to be even less grounded in reality than if he'd fought his way up from the mean streets.
Imagine what Hitler's son might have been like.
You are correct. It was inherited. But nothing in this regime is guranteed, even when a bloodline is involved. He was not immediately made leader after his father's death in fact. It was three years later that he was made the head of the Communist Party.
Kim Il Jong was a despot in training. His father groomed him for the position but this grooming involved him being trained in the art of intimidation and the reminder that someone was always looking over their shoulder. He was paraded from millitary post to police station in his 20s and 30s and trained by his father in intimidation tactics. His ascension was more formal in nature, but the rules he grew up with are functionally the same rules of the street.