Lady Luci did not need any special reason why she had to shit on this guy. She did it because she could and that was reason enough for her. Lady Luci cruelly used her shit to punish the guy and when he tried to ask her why she was doing it, she gave him a cooked up reason but the real one was that she did it because she could.
Lady Luci had to punish this guy in a manner he would never forgive for as long as he lived. He was straight but she used her fisting fetish to nearly tear his ass apart. But she was not done with him as she also had him eat her shit as well as drink her urine. The guy was shocked by the cruelty but at least he learned from it.
Lady Luci wanted to piss off her slave and so she used her shit to do it. The slave deserved it because he was not being a good slave and she felt it was important to show him what would happen to him if he continued behavior that way. He got scared of what she did to him and that is how he made the adjustments that she wanted him to make.