Both sides did it. This had been normal in that part of the world for a long time. The Communists killed a lot more by at least an order of magnitude - any peasant with more than two pigs, etc.
Both were pikers compared to what the Japanese did to Koreans during the 50 years of Japanese occupation. The Japanese were outright evil. The Communists and Rhee's ROK just wanted power. The Japanese did it for fun.
The Japanese industrialized North Korea and Manchuria. They wanted the place so they could keep the home islands environmentally decent.
They did attempt to exterminate the Yi, or top noble family ~ but I've met a few so they didn't get 'em all.