Well, they are assholes, and the idea that they're randomly malicious about it would be our moral scales imposing upon theirs. They're raised and trained to be assholes (and by extension, merciless) by their government that operates in the same way. It could be argued that they need a reason (no point in killing an obedient adult when they can be forced to work instead), but honestly, if NK actually had the resources to occupy somewhere, that's how hardline soldiers might act initially. They are fanatically nationalist, especially against Americans and South Koreans, because the Kims have been saying that every ounce of suffering in Korea is because of America and SK for decades, and the layman has no way of knowing otherwise. However, while occupying places, the soldiers would be exposed to different cultures, information, and ideas, and dissent would flow within the ranks within months, even faster than within the Nazis.