Mistress Lilli did not like how clueless her slave was. She went out of her way to teach him how to stop being clueless, but he did not really pay much attention. That pissed her off and she had to teach him a lesson. She did this with her facesitting fetish so that he learned his lesson the hard way. It was brutal, but there was nothing he could do about it.
Mistress Lilli had a disagreement with her sugar daddy that she had to teach him a lesson and make him realize that she would not be taken for granted. That is why she did what she did by facesitting him. She made sure that he was in pain and that he was humiliated by what she did to him so that he would never forget it. That way, he would never mess with her.