Well, going with my hunch that NM and SH had decided leading up to that day that they were going to kill Becky in a wood, and stage it as a suicide, I am 100% certain that there was a kidnap plan. There is no other way they could get away with taking her away and her remaining alive. I do not believe that an argument over money or untidiness or attitude led to an unplanned killing. I think the purchase of the stun guns seals it.
As the plan was to kill, it didn't take much of a leap to have to do it when Becky fought back and they were thwarted in their efforts to get her in the car alive.
As regards a sexual motive, I am wavering. I do think they may have intended to have their fun with her before killing her, and I do think they have the required perversion of mind, to take enjoyment from controlling and dominating another.