That's a very good point. I tend to avoid that language in general for exactly that reason, because it's hard to manage all the nuance needed to express the concept of people behaving monstrously.
That said, the greatest evil requires not just characterizing other people as monsters, but believing that monsters aren't people. And really, any kind of "those things aren't really people" will do it. Recognizing that the other people who are monsters are still people is very important.
Selectively denying that people are monsters only because they're like you, or part of your heritage, though, is a very important step towards being comfortable thinking that monsters aren't people. If you have to admit that "monsters" is a category which includes people you care about, you are going to be less comfortable with dismissing the humanity and rights of the "monsters".