The guy thought it would be fun to challenge their authority during one of their interactions. He was their junior, but the smirk he gave them when they told him to stop was all they needed to see. Mistress Gaia and her friend did not entertain rebellion. Their rule was absolute, and his mockery had gone too far. They had to crush him and put him in his place, which is what they did by spitting, peeing and shitting in his mouth.