I think it is a certainty that whoever or whatever does the mutilations is aware that carved up human corpses are going to bring the heat down fast and hard. If they are doing it to people, they probably dispose of the bodies in a way that they wont be found or look like they died from other causes.
Cases I read about are only like 2 or 3.
But if some of our visitors see us as little more than animals(some contactees over the decades reportedly have been told this), then it stands to reason they may treat us as such as well.
After all how many humans die by other humans every single day worldwide, via murder war or? Think of how we may look to a visiting species where that has not occurred for them in thousands of years.
Some years ago I read a case where a woman that had been abducted saw tanks on the ship with human body parts in them. Are these the remains of some those that go missing never to be found?