Inconsistency is something mistress Dula cherishes. She was not ok with how inconsistent this slave was. So she had to do what she does best and that is how she punished the guy painfully. He tried to beg her for mercy but she ignored him as she was having fun doing it. In addition, it was sending a message to him and he got the message as she wanted him to.
Mistress Dula did not want to let her uncle continue being bigoted. She wanted to teach him a lesson and she did so using hard slaps. The mistress slapped him while he thought that they were enjoying themselves and having a good time. But he realized that she was not and she humiliated him by slapping him to make sure he never did that again. He toned down his bigotry.