Have always wondered about the killer actually being female ...re-watched the opening sequences of Ep.1 ...wavy dark hair that could be long (scarf wrapped around neck), only seen hands with golves, slight figure when seen 'fighting' Martin, someone who will 'do what it takes' to get their wishes!!
So ...I'm thinking "Charlotte"! 
Well ...there is that whole wig wearing issue...which she wipped off for a quick encounter with Martin; the whole manipulation of the medical system to get her husband's operation; the eventual paying of the money and not killing Martin ...and yes, she could have an accomplist (or two) ...one of whom, for the final twist, could be August (maybe she's the 'Freida' on his computer??)
What does anyone else think?