No denying that the South was very nasty from the 1950s through the early 1982s, from Rhee to Park to Chun. Purges, massacres, and even a nuclear weapons program. However, trying to draw too many parallels doesn't work either because the patron states behind both regimes had very different approaches. Given Rhee's unsavoriness, fear of a Southern led invasion triggering global war, and the broader political instability in the late 1940s, the United States never really bothered to equip the ROK with the heavy weapons needed to wage an offensive campaign. At the start of the Korean War, their forces were pretty much a glorified gendarmerie, and the United States held the leash tightly to prevent a war. The Soviets on the other hand, had no restraints and fully equipped the North Koreans with the latest heavy weapons and green lighted an invasion.
Even as you go through the Cold War, the United States played a very careful balancing act, trying to prop up the South while actively constraining them from launching a reunification campaign (that could spiral into WWIII) and actively squashed any efforts by the South to become a nuclear state. The North has always been much more openly aggressive, maintaining a forward positioned posture and threatening invasion at every turn.
There's also still no overlooking that the South has evolved into a relatively liberal, democratic society that is a responsible global player. Whereas the North is still very much an old school totalitarian dictatorship which continues to flout international norms.