Any ideas as to what this might be?
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Mmm. First off I think the kidnap was definitely a part of their plan. He had all the kit/tools with him - unless he always kept in his boot just in case. I doubt they would have attempted a sexual assault in Becky's room.
But, having said that, is there any proof he actually did have any of those things with him? Let me think, the mask (no), the hand cuffs (no) the stun gun (poss- they bought batteries on the way) tape (no) suitcase (no). I can't see a kidnap happening with just a stun gun and nothing to restrain her with.
So I do think it was a kidnap plan. I think on balance they would have taken her to some remote spot or wooded area, rather than their home. And perhaps they were just too stupid to think through how it would end. Perhaps the black bag was to place over Becky's head but she would see them approaching her with it. Mask or no mask, she would know it was him. She would hear them/him talking to her.
Was it for rape? Was it for revenge/violence/a 'lesson' to scare her?
Unless he gives a true confession I doubt anyone will ever know his/their intentions. It's just too hard to put yourself in the mind of someone like that. He seems to be a perfectionist in terms of his wrapping up and decontamination. So it is hard to think that he didn't actually have a plan after kidnapping her.
As far as what I think may actually have happened to Becky - a very violent and prolonged restraint and suffocation involving two people pulling her around and pinning her down. a hand over her mouth because she was screaming, to stop their daughter hearing anything. I think the final pinching of the nostrils was a deliberate act to kill her when SH flipped. Somehow I think NM had more of a hang-up, for want of a better word, about not causing really serious harm, than SH. I think she is the cold psychopath if there is one between them. I do see him suffering emotionally, I don't see an ounce of it with her.