I think Adele killed Robert and what she's actually holding over David is that he dumped his body down the well to protect her. I think she may also have started the fire that killed her parents. I think she bought the heroin to overdose Louise (the last we saw of her was when she was in Adeles house thinking she had OD'd. I think she probably did similar to the woman in the cosy cafe in Brighton.
That's all very predictable so I hope I'm totally wrong. The only bit that's a bit weird though and making me think I might be wrong is that in Louises nightmare it seems like she's in a burned manor house like Adele's family home which would be odd because she wouldn't have known about that house to have nightmares about it or maybe she's predicting the future and Adele will bring her there I don't know. Also what's the story with Adele dreaming she was in the room with David and Louise and that's how she knew about their affair hhhmmmm
Anyway good show so far, I hope the ending doesn't let me down