Why does Miss Merle slap Janey, and how does this make you feel after reading the chapter narrated by Merle in A Gathering of Old Men?

In A Gathering of Old Men, although slapping Janey is rather cruel, Miss Merle realizes that the servant is hysterical and she needs answers from Janey immediately.
In the beginning of Gaines's novel, the conflict is established early on as the Cajun farmer Beau Boutan lies dead, having been shot by Mathau. This situation is dire because the Boutan family has a history of violence and Beau is white and Mathau is black. Candy Marshall is white and from the family that once owned the former plantation; nevertheless, she claims she has shot Beau in order to protect Mathau, who has been like a father to her.
In Chapter Three, Myrtle Bouchard (also called Miss Merle) has a pie made so she can take it to the Marshall's house where the Major resides. After she arrives at the Marshall home, she observes that the servant Janey has been crying. When Janey tells her about the murder, Miss Merle tries to awaken the drunken Major on the porch, but he is passed out. As a result, she drives alone to where the black people of the community live -- in the former slave quarters. There she observes Mathau, Johnny Paul, and Rufe sitting on the porch holding shotguns. She sees the bloodied body of Beau Boutan, and Candy Marshall tells Miss Merle that she has shot Beau. Miss Merle does not believe her, yet Candy insists that she committed the murder, adding that the three men have also confessed to the crime. Candy asks Miss Merle to ask Janey to get the names of as many men as she can that do not like Fix, the father of Beau, and have them come with shotguns. 
Miss Merle returns to the Marshall house. She asks Janey who does not like Fix. The terrified Janey says that she does not know anyone who does like Fix. Then, Miss Merle asks her if these men will stand up to Fix. Janey cries, "I don' know what you talking about." Miss Merle explains that Candy is going to protect Mathau by having as many men as she can come with shotguns and have them all claim to have shot Beau. Their guns will have one barrel empty, so she needs used 12 gauge shotgun shells as proof of this. 
"Get them on that phone!" Miss Merle orders her, but Janey begs her, "Please don't make me do nothin' like that." Miss Merle slaps Janey and orders her to get on the phone. She admits that she took her frustrations out on poor Janey, but narrates that if she has to slap her to make her understand that she must get involved, she will. The situation is dire, and Janey is the only one who can contact these men. Unfortunately, Miss Merle feels she has to slap Janey to get her to think sensibly so she will obey her. Sometimes when people are hysterical, it does become necessary to give them a little shock back into reality. Often someone will shake a person; however, Miss Merle perceives Janey as a servant and is crueler than she should be.