Sadistic bullying scum indeed. But they keep away from the power that be (human) and only torture the thieves out from human's sight, so likely human won't give a damn.
It's debatable if they are really scum or just the paralel of a hitman/mercenary (which if they are scum or not depends of the jobs they accept, at least in fiction).
The story seems to aim for an absolute grey/dark grey, since while clearly sadistic, the Kimeemaru's shown as necessary since the average yu's are shown as unreasonable and self absorbed, somehow without evil intention, but one may argue that could be the narator being unreliable. There is even the implication that their next hit maybe a rapist Alice, at least when you consider why an Alice "making trouble with local pet's" would stand out so much like for the neighbour town to hear about it. Plus the Kimeemaru's are shown having a surprisingly reasonable mindset about how to make deals with humans, something RARE in yukkuris (their usual concept of deal usually is "I GET THE GOODS, you give them to me without questioning because -insert worthless here-!)
Maybe this will change when Humans become more strict and stop yukkuris form abusing of their hospitality, even if it means squashing them constantly, This HitKimee's wouldn't exist if the humans in the story weren't soft and gave a proper middle finger to the Yu's.