So, what about the killer and her borderline personality disorder? Originally, borderline was so called because it was a disturbance bordering on psychosis. Right on the edge. B.P.D.s form immediate attachments. They idealize the other person and their feeling is intense. The connection becomes stormy if Borderline perceives a slight or a separation occurs or a conflict arises. Emotions shift to intense anger & dislike.
We can see this in the killer's relationship to Nurmi, where she fought hard to keep him, he was "the only one" who could prepare her case & represent her. By contrast, look at them now. B.P.D.s view the world in extremes: all good or all bad. We've seen her relate to her family members in these same modes. Of course, it was conspicuous in her treatment of Travis Alexander.
Borderlines are prone to unstable moods that last for hours, as well as episodes of impulsive aggression. Their actions are unpredictable. There is distortion in their sense of self, evident in Arias, who is a narcissist. Their goals change, as was apparent when she was following The Law of Attraction and The Secret. Values undergo change and she professed such reformation owing to her Mormon studies & baptism.
Cognition, i.e., perception, reasoning & intuition are all gnarled & deformed due to her perspective on the world. In jail, she is being given drugs for this and to stabilize her moods. I would have included the explanation of her disordered personality in the post on psychological mitigation but I was running long as it was. Also, I realize Sleuths bring their own opinions on Borderlines to the topic.