When Schoolteacher finds Sethe, he blames her actions on the nephew that beat her. He thinks that Sethe, like an animal, was led to run away and kill her children because the nephew "overbeat" her.
Section 1.16, page 149-150 "But now she'd gone wild, due to the mishandling of the nephew who'd overbeat her and made her cut and run. Schoolteacher had chastised that nephew, telling him to think -- just think -- what would his own horse do if you beat it beyond the point of education...See how he liked it; see what happened when you overbeat creatures God had given you the responsibility of -- the trouble it was, and the loss."