Can it really be so hard for writers to understand that supporters of slavery and genocide could rationalize their own actions in the same way?
The problem is that the writers usually come up with massively fake-sounding arguments when they don't base their arguments for the characters off real-world sources. It's on a par with villains who knowingly think about themselves being evil.
I tried this in one of my stories. Not knowing much about modern slavery, I just made the argument.
The society was formed out of artists/philosophers/scientists after the collapse of an empire. As such, they value logic, and see purity as the ultimate expression of logic. Therefore, the majority (which supports slavery) sees half-breeds and half-creatures as inferior, because they are impure... Since their society is dedicated to upholding logic, and these people are inherently illogical, they must be inferior.
I don't like the argument, but until I do more research I can't come up with anything better.