The slaves were treated like animals, unfortunately, and because of this, many of them were conditioned to believe that they actually were animals in many ways. They were tortured, abused, and used for whatever purposes the Caucasian race wanted:
Because slaves are treated no better—and sometimes worse—than animals, it leads them to question what it is that makes one human. While Mr. Garner was alive, for instance, Paul D. truly believed that he was a man. But after the schoolteacher arrives and puts the bit to him, he learns a different lesson: "They were trespassers among the human race." (Enotes)
Also, slavery turned the owners into "monsters" (Enotes). The owners became hardened to their slaves and ceased to view them as humans who had feelings and emotions, etc. They were desensitized to their own brutality, unfortunately. Baby Suggs says that "they [the owners] could prowl at will, change from one mind to another, and even when they thought they were behaving, it was a far cry from what real humans did" (Enotes).