I know I'm a bit late to the partybut maybe I can provide some resolution on this one.
Just finished watching the final episode of WTD. Read this thread, and went back and watched The Hardest Word.
Grete didn't kill anyone. She was a victim with sex issues due to her abuse by her father, but she was not a murderer. She'd have had no reason to kill Dave, because she didn't even know what the murders of the men she'd slept with meantshe was completely overwhelmed when her father confessed, and probably had no idea who was doing it up until that point (she may have figured out it was him on some level, but hadn't consciously come to terms with it), and so there was no reason to try to cover anything up by killing Dave. Certainly she wouldn't have killed a good man and a fine cop, the "one person she trusted," to protect her father, even if she had figured that out.
We weren't supposed to know who offed Dave the cop, but he was making a connection regarding Penn & something with the Barac crime organization; they did reference in one conversation after going through his files that it was probably someone in the crime family that offed him because he was getting too close, but not Penn personally, as they had him on surveillance at the restaurant at the time of the murder. Dave had started to be suspicous about sex trafficking by the org. after the asphyxiation crime scene, when Andy was playing dumb in the flashback & more interested in his own reflection in the mirror. He probably was starting to think that something about the scene was set up to cover up that connection; meantime, Andy was protecting Penn by pretending not to make the connection. I presume the killer was following Dave, looking for an opportune time in an isolated place and just happened to pick when Grete was meant to meet him in the carpark to take him out.

I also think that Greta killed the cop.

I assume Greta killed him and that Boyd was being unusually kind.
Many times in life things aren't answered conclusively and so it is in this programme. I like the fact that they leave a little bit unsaid about some cases as I enjoy engaging the grey matter.

I was confused about who killed the Dave guy too and also at the end, Boyd says to the daughter something along the lines of "see you tomorrow, we've got more work to do"was he just being kind? Surely she wasn't going to work on that case (or any other for awhile, reallywouldn't she be charged with something?) There have been other episodes that seem to end without explaining things fully and that is frustrating to me. In some instances I can think things through and figure it out, but in other instances I feel a bit cheated because I've invested two hours in the episode and I want to know.

I remember The Hardest Word being confusing at the time too, there were some great performances in it, especially from Paul Reynolds and James Dreyfus, but the plot was hard to follow!

Glad it helped!

Thanks so much for the link! I'm still mostly clueless but at least I'm not alone so that's some comfort.

They never revealed Dave's killer. That whole episode was confusing.
This summary may help
http://www.thatboardthingee.com/evidenceroom/waking-the-dead-tv/whatwh y/17/the-hardest-word.html
I can't say I really liked that episode. It was a little off. Not what I expect from a WTD episode.

I remember this episode. Father and daughter were definitely not working together but as I recall she should have seen it and withheld information, either because she was in denial or because it would have affected her own career.

I agree!! I just watched this episode and feel very confused. I came here hoping to find some answers. Were father and daughter working together? And who killed David Marvin?