How does Sarny react to Waller's treatment of Nightjohn and Mammy?

In the book, Nightjohn teaches Sarny how to read and write. Sarny is so excited about learning that she begins to write on the dirt in her spare time. When Waller catches her etching the word "bag" in the dirt, he demands to know who's been teaching her how to read and write. Sarny is too afraid to tell him, and Waller physically assaults the little girl.

Frightened, Sarny runs to the slave quarters to seek protection at Mammy's side. Waller confronts Mammy at the slave quarters, and when she refuses to shed any light on the situation, Waller drags Mammy to the spring house. There, he shackles her to the wall. His plan is to wait until the field slaves return in the evening before he flogs Mammy. Waller enjoys making all the slaves watch his brutal whippings.

According to Sarny, Waller strips Mammy naked and hitches her up, in place of his horse, to pull his buggy. To get her to move faster, Waller whips Mammy's back mercilessly, as if she were the horse. At some point, Nightjohn speaks up in defense of Mammy. He tells Waller that he's the one who taught Sarny how to read and write. Waller doesn't whip Nightjohn, but he forces two field hands to hold Nightjohn down as he chisels off two of Nightjohn's toes (one from each foot). 

Sarny is furious after witnessing Waller's treatment of Nightjohn and Mammy. She is grieved beyond words that two of the people she loves have been treated worse than animals. Sarny begins to wish for Waller to be put into a hell of fire and brimstone, where there will be devils who will torment him mercilessly.

She wishes for Waller to experience the kind of hell Mammy has told her about, where devils will tear the skin off the backs of evil people, eat their defenseless victims, and disembowel those who deserve the worst kind of hell there is. In short, Sarny is angry and in deep grief; she wants Waller to be punished for all the suffering that he's caused Mammy and Nightjohn.