"hideous primal instinct" that's right, ain't no one wanna deal with that ever, nuh-uh, no ma'am
Well, I think she's right most people wouldn't want to deal with a partner who is barely restraining themselves from killing you. That's the implication of taking mana from someone dealing with mana sickness.
It explains a lot of Rachae's behavior. We see that she is responsible, so, for instance, if it had been a full moon she might have run away instead of taking care of Lin Xiya. Lin Xiya wouldn't literally die from being left alone with mana sickness, so it would be a reasonable choice from Rachael. However, from the other perspective, it would seem like Rachael abandoning her in her time of need.
Likewise, Rachael doesn't speak kindly a lot of the time because she doesn't feel she is acting kindly. She has a mental block around "acting like a nice person" toward Lin Xiya because Rachael would feel hypocritical. Rachael has even killed a lot of people, which doesn't usually get taken seriously in these types of stories, but in real life people often have some level of trauma over that. Even if we disregard the war trauma angle, it makes sense she is dealing with a lot of guilt over her behavior.