JMO
At least right now, I guess I am thinking they may have just tried to rob her and something went wrong where they felt they had to kill her. Like maybe they hoped that SL would never be able to identify them and maybe they realized SL would know how to find out where they lived or who they really were so maybe they felt they had to kill her to keep from getting caught.
Or maybe SL began to put up a good fight as she was being robbed.
The reasons I tend to think robbery is because the POI ended up at a casino if I read things right. I could see them going there if they ended up getting some funds from their victim.
The other reason I think they were trying to keep their true identify from SL was because of the strange videos they released. It showed that they were thinking to stay disguised. So I am betting they wanted to stay disguised from SL too.
I think SL probably only met with the female and I bet the male POI was not part of the initial meeting. I am kind of thinking right now that the female hoped to have robbed her and just run off and maybe the male person was close by if anything went wrong.
Just speculation as to the motive. I dont get a sense they had a very elaborate scheme whatever it was. Although I would not doubt if there are other victims of these two. They may have done this sort of thing before and maybe were getting better at it until they ran into SL.