I think this was sexual from the start and never about kidnap. NM was interested in Becky, perhaps even obsessed. SH was jealous of his interest (and being pregnant with twins was probably aware that she wasn't as attractive to NM being bigger than usual). I think she was probably happy for poor Becky to be violated, as she was jealous of her for a number of reasons. In fact, she hated her. I think NM was jealous of the relationship that Becky had with his mum too. (Maybe he thought his mum had given Becky a better childhood than he had?) I think it was a mixture of attraction and jealousy that caused NM to do what he did.
I think that when he discovered that he couldn't do what he wanted with Becky, he got angry - as he was too far into the plan to back out of it as obviously Becky could tell on him. So, angry for failing to get what he wanted, but having all the problems of gettind rid of her, without the 'fun' he thought he was going to get.
Like others have said, I think SH may have smothered her, rather than NM. Maybe by accident at that point, to keep her quiet or still.
Either the houses next door were empty at the time, or their neighbours are hard of hearing, as I find it hard to believe that nobody heard a thing.