Sarny doesn't like Clel Waller at all. And it's not hard to see why. He's the plantation owner and she's a slave. And like most plantation owners, he's a brutal man, a truly nasty piece of work who treats his slaves worse than animals. (The slaves are even forced to eat from a trough like pigs.) Moreover, Waller uses dogs to hunt down runaway slaves, allowing them to tear flesh and inflict terrible pain. In his cruel and degrading treatment of the slaves, Waller shows that he regards them as less than human.
Sarny witnesses at first hand just how vicious and nasty Waller can be. The man has no redeeming features whatsoever, and so the only appropriate emotion toward him from any slave is one of hate. And Sarny expresses her hatred of Waller using such colorful insults as "nothing," "dog droppings," and "pig slop."