I think it is a combination of the two things. I think he was planning , in his mind, for awhile, to set up and stage an abduction scene. And he planned, at some point, to dispose of his bothersome family.
But then that night, SW may have said something, about knowing he was seeing someone else. An 'emotional conversation' happened and she was taking the girls with her, to stay with friends, while he figured things out.
URGENT ALERT. If she left with the kids, and told people they were separating, he could NEVER carry out his plan. Once she left, there would be no way he could come home from work and say 'Hey, where'd my family go?'
Once she left, she was going to file for divorce, and his financial nightmare would begin.
If he let her leave that night with the girls, then he would never be rid of his family and the burdens and responsibilities they had become.
JMO MOO IMO