Addressing this first, all states are authoritarian, the difference between them lies in their class character, ie are they working class states, or capitalist states. All states are instruments of class oppression, solidifying the power of one class over the others, and in the ROK that class was the capitalist class, especially the former compradors that benefited from Japanese colonization of Korea.

Going forward, even without armed resistance, Rhee Syngman was still slaughtering labor organizers. The cause was the labor organizing, not the resistance to him. He was hand-picked by the US Empire specifically to be used as a tool to wipe out any semblance of a Korean left south of the parallel. You’re putting too much of an emphasis on the resistance, and not on the root cause: leftist organizing in general was unacceptable to the US Empire and the comrador regime they installed.

The fact of the matter is that daring to organize is what caused the massacre. The headline doesn’t have the full context, not even the full Wikipedia article does. I suggest you stop caping for Rhee Syngman and instead try to look at the root cause, which the OP does get closer to than what you’re saying.