It is indeed such a puzzling case. At the beginning I didn't believe the kidnap plan, and thought it was something NM had invented. Now I have changed my mind. Why would he invent something so unlikely as to be almost unbelievable to explain how he ended up in a struggle with Becky? The texts between him and SH show this was something they had considered carrying out together. The stun guns (two of), handcuffs and mask are inexplicable without a kidnap in mind. A rape in her bedroom? - no too much risk for contamination of the room, and being disturbed by Anji coming back early or her bf calling round. So that leads me to think it was their plan to remove Becky all along and I can't see that ending in any way except with their capture - she would know their identities. So, did they plan to kill her all along? It's the only way they could hope to avoid repercussions.
come to think of it, NM hasn't admitted to trying to kidnap Becky, so I don't know where I got that idea from!