The AP article ignores 2 important pieces of info:
The NKs killed American prisoners as well as S Koreans, frequently torturing them first. These bodies were found when the UN troops broke out of the Pusan pocket.

Second, there was an ongoing guerilla war going on in the South throughout the Korean War. (There was a home-grown Communist movement in the South that predated the Soviet occupation of the North.) These executions should be seen in the context of ongoing attacks accompanied by atrocities by the guerillas.

Neither side was interested in taking prisoners.

Al