By 1950, the U.S.-backed Syngman Rhee government in Seoul frowned on communism. And by "frowned on," we mean they imprisoned 30,000 people and sent 10 times as many into re-education camps, called the Bodo League. At one point the North managed to advance the front, and the Rhee government had to retreat. That left one problem (or, like, a quarter million problems, depending): the prisoners. The Rhee didn't have time to relocate them or money to feed them, so they took the Metallica route and decided to just kill 'em all.
Prisoners were taken out of the camps en masse and executed without trial. In Busan, about 10,000 were bussed out to the city limits and machine-gunned. One admiral told the Associated Press that he had authorized ships to take prisoners out to sea, execute them, and dump the bodies. He described the killings as "common practice."
Major Abbott/U.S. Army
We don't have info on blimp executions, but we wouldn't be surprised.
The U.S. knew all too well about these atrocities, and while they weren't exactly handing out high-fives, they also didn't do anything to stop them. Worse, they reportedly helped with the cover up. It wasn't until 2005 that the truth finally came out, but by then modern North Korea's antics had stolen the limelight, claimed they invented the limelight, accused the limelight of treason, and had the limelight fed to wolves.