It absolutely is. It's a thought process people use to make their actions easier to justify to their own sense of morality and empathy. Militaries drill that same thought process into soldiers for the same reason. It's much easier to shoot something than it is to shoot somebody, so your enemy must be less than human to make pulling the trigger easier. So you turn a group of people into the Other. And the more you do it, the easier it gets, and all it takes is somebody convincing you that the wrong group deserves the same treatment.
"My enemy isn't human. My enemy is less than human. Since the dawning of Mankind, this has been the battlecry."
Also, I take issue with the entire concept of "sin," so that argument holds no water for me. I mean, what a meaningless "argument". How does it absolve them of anything??
I don't care one way or another about the theology around the word "sin," but it's the perfect word to describe their actions. By reducing them to subhuman, you make it easy to remove their free will in the equation - it becomes the "just following orders" excuse - and it's incredibly important to remember that they chose to do this. Nobody is born hateful, hatred is learned. They made the conscious choice to be hateful and act this way, and they should be held accountable for that choice.
People have a hard time accepting that the worst people this planet has ever seen are as human as you and me, but to deny it is to turn a blind eye to the fact that the people in that video could be our neighbors, friends, or family. Because we know them, and they're good people, so they'd never do something like that, right?
You're thinking of Gandalf Big Naturals. Easy mistake to make.