I think the only thing Ros was guilty of initially was only looking at the evidence that fitted her theory (eg ignoring what Tim was trying to tell her, ignoring Michael's alibi etc). I think it was one of those tropes in TV copland (& in real life, hence miscarriages of justice) where the genuinely believe the guy did it, so a little massaging of the facts won't do any harm...
I also don't think she killed Tim, but that he probably cut himself with the saw out of shock after she opened her eyes. If she did push him off and the saw cut him as a result I still think it was an accident. Up to that point she's not particularly bad.
However... She decided to cover up a death, mutilated a body, has tampered with physical evidence more than once now and is clearly now in it up to her neck. She's completely done for so she literally has nothing to lose by going deeper and deeper.
I think the husband is a red herring, but will possibly turn out to be a witness to her being at Tim's flat.
Maybe another body's going to turn up, and with Michael in custody and Tim dead then that's going to really throw the whole investigation up to this point into doubt and what will she do then?
Side issue, anyone else find themselves struggling not to see Vicky Mclure as a scheming psycho after her performance in The Replacement? I keep having to remind myself that it's a different character and that everything she says isn't intended to mislead
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