I wondered if he tried to force SM and bondage and she told him no. In a rage he killed her.I have had that thought as well and tried to figure out the kidnapping thing. I wonder if she tried to leave once they got to his place and that is where the kidnapping came from.
Fire was logical to get rid of her huge bag of clothes and luggage. I think doing the same with her was an after thought alternative.
The irony, was he was already doomed by the lyft driver and the phone.