You don’t know Vivian’s backstory and are judging her. She already said they invaded her territory, was she suppose to give her neck to them. Adventurers can kill but not the victim, that’s very self-entitled. Also it’s very common to become a slave if you lose in a life and death situation, even human experimentation or torture wouldn’t be far off. Their common sense is different from ours, first understand their perspective before coming up with an opinion, an opinion organized factual evidence based on human psychology that would prove your statement.
She also most likely didn’t choose to be like that. She cried when she finally became human. Can you say for certain you know what she went through.