Yeah, that's right, a hat is making us go ick. Specifically, the "black hat wide-brimmed" (26.145), worn by schoolteacher and Mr. Bodwin. The hat isn't just a handy accessory that keeps your skin from turning into the surface of a wrinkly, burned prune. In Beloved, it also means status, power, and—you guessed it—evil.
But here's the thing. The hat is really only important to Sethe. She's the one to see the shadow of the hat first, before she even sees schoolteacher entering Baby Suggs's yard. It's the hat that propels her to take her kids into the shed and attempt to kill them—she knows it means trouble. The hat also drives her into a wild frenzy at the end of the book because she just can't let go of her negative associations with this creepy accessory.
Sethe knows what real slaves knew back then: the hat was an accessory typically worn by slavemasters, not by slaves. (Slaves, by the way, usually wore headwraps to cover their hair. Here's why.)