Mistress Medea was out to control this guy and she knew that the best way to do it was to use her control over him to torture as well as humiliate him. That is why she used her anger to do it. He was asked to lick her saliva as well as to lick her dirty feet and he had no alternative but to do as the mistress had ordered him. And from how he was scared of her, she had an easy time controlling him.