Madame Marissa was not ok with having a slave who was a coward. That is why she went out of her way to trample and humiliate him. The mistress had to make sure that he realized that being a coward was not acceptable to her at all and that he had to stop it and find a way to brave. So she sent him a warning shot by humiliating him using her feet. He had to lick and smell stinky feet and socks.