at around halfway through episode 3 or 4, I figured it was his wife for sure. Tommy is the obvious red herring, being super unlikeable, jealous, and having a thing for the victim (which they don't spell out until the last episode). I was wavering a bit and just assumed it was someone in his family towards the end, mostly because of the way she was tied up and I figured it'd be that he left the case files around. But the wife still seemed like prime suspect.
It's only been like 30 minutes since I finished, but they really didn't stick the landing. The more I think about it, the worse it gets. Like, for one, apparently the entire fucking family decided to visit Caroline on this one night and somehow with 4 visits (him, son, daughter, him again) they all managed to avoid each other. But then his 2nd visit makes no sense, because I swear they mentioned using his phone to pinpoint when he arrived home ("almost 11pm on the dot" was the line), but wouldn't they have seen a 2nd visit? Did he just randomly leave his phone at home for that one?
Then you get to the whole act. First, they mentioned repeatedly how he avoided getting blood on anything, and yet, he rushes in the house and starts cradling her dead body...and manages to come away clean? Then, where the fuck did he get the gloves and rope to tie her up without leaving any clues? They threw out a quick shot of rope when they raided his house on the warrant, but surely he didn't go home to grab supplies and it was actually 3 trips? I know they made it a point to say that it was generic rope anyone could get, but I imagine they could've gotten a close enough match if it was the same. If not, surely a trip to the hardware store would've popped up on their radar. And then somehow the teenage daughter is crafty enough to clean up her mess in the car without a police team finding a trace of any evidence. A miracle!
All in all, I think they tried to get too cute with the twist. I don't know why writers have such a hard time with it, especially when the rest of the show seemed so well thought out. It almost makes me think it wasn't their original plan, cause they should've accounted for all that stuff to make it believable. The only hint they did was the "the murder seemed like a crime of the moment, and the tying up methodical." I feel like they should've just gone with one twist instead of 3 (accusing the wife, saying he tied her up, and then pivot to the daughter) and it would've made it more manageable.
