You've hit on exactly the point I try to stress with my students: if we make them out to be evil, what we're doing is divorcing ourselves from them morally, turning it into an us-and-them situation, where "those bad people" did something WE would never do. I believe that everyone has within them the capacity both for great good and for great evil, and that great evil can be done when a few people do horrible things and everyone else stands back and does nothing. This is a fundamental concept of citizenship, and to negate it is to reduce the level of responsibility that we need to take in our society.

So, by branding all slaveowners as inherently evil and allowing that perception to stand in my class, this person has unwittingly created a bunch of bystanders to modern evil.

Oh, that's a sweet thesis. I need to write an essay about that one.