I think BT's disappearance was pre-planned, and happened the weekend before she went overseas alone to be with her family for a reason.
The timing of her disappearing right before a major trip to be with family, when she had no family or friends locally, has always stuck with me.
Just like the timing of a visit with a family member was coming up for Erin Corwin, who had no family locally when she disappeared in the desert in SB County the week before her mother was coming to visit her in Twenty Nine Palms. She had a 'mighty secret' (she was pregnant) only known to her and her (married) lover, who killed her right before her mother came to visit. Although he later confessed he did it to keep his wife from finding out about their affair, her death was pre-planned by him, presumably, IMO, to make sure that face to face upcoming time with a close family member would not occur during which she might let the cat out of the bag.
In BT's case, I think there was something about RT or his activities or some unhealthy aspect of their relationship that he suspected she was going to share with her family while away from him that he decided he didn't want to risk her sharing with them.
ETA: So he planned her disappearance, either by himself, or with the help of others, or left her somewhere vulnerable where she would be disappeared by others without directly implicating himself. Time will tell, I believe LE is making progress in this direction, and hope Barbara will be found soon.
MOO