Goddess Kitra wanted to alienate this guy and she felt that the best thing to do to the guy was to use her shit to do it. It was easier than anything else she could think of and that is why she did it. It was fun for her but it was not for him and she knew that she had to do what she did. She did not want it to end.
Goddess Kitra wanted to dominate this mechanic because she did not like the kind of services she got from him. She did not want others to also suffer like she did and that is why she had to punish him. The mistress knew that the best way to do it was to use her shit and that is why she did it with her poo. The mistress fed him poo and changed him.
Goddess Kitra did not want a slave who was too loud for her. She tried to raise her concerns with him for him to change but he did not and that pissed her off. She did it a second time and when he did not take any action about it, she knew she had to act and she did this by turning him into a human toilet and feeding him shit.