Sometimes evil looks a lot like a vanilla wafer. And that's schoolteacher—a boring, benign-looking academic-type, glasses and all. Scratch the surface, though, and you've got a prototype for Dr. Evil—and then some.
Schoolteacher doesn't appear threatening, which is why the Sweet Home men don't take him seriously at first. But Morrison doesn't let us make that mistake. Think of this scene, for example, which should totally freak you out. Sethe is recalling the day she got raped by two boys at Sweet Home
"I am full God damn it of two boys with mossy teeth, one sucking on my breast the other holding me down, their book-reading teacher watching and writing it up." (7.70)
And there you have it: schoolteacher in a nutshell.
What is evil here? It's schoolteacher's ability to view Sethe as a thing, an animal to be coldly observed even when it's clear that she's suffering something totally inhuman. In other words, schoolteacher doesn't have the ability to feel compassion or empathy for other human beings. And that means he can be capable of all sorts of cruelty.