Re: the affair, I think we are supposed to be focussing on how people who think there is no way out (John, who couldn't see any way out of Vicky blackmailing him, and wasn't ready to leave his -he thought - dutiful wife) will go to extreme measures.
He was wrong, of course. There was a way out, as his wife was having an affair too. The dutiful wife, who he thought would be devastated, actually couldn't have cared less.
He same reasoning is why I don't think Daryl killed the prodtitutes. He obviously has some significant needs, and his mum has always taken care of him/solved his problems/ found a way through whatever scrape he got in. But this one (killing the prostitutes, a conclusion she jumped too as it was never actually said - lots of talking at cross purposes, and tangents) was unsolvable, except by what she did. There was nothing more she could do, she felt she had failed him, and so she (in Mice and Men style) made him as happy as she could, while doing everything she could to make sure he didn't go to jail - not escape scot free, but she knew he wouldn't survive jail without her. But I think it will turn out (as with the affair situation) that there was another way, because he didn't actually kill them.
So both Farmer mum and John will have ended up killing for no reason, whilst thinking it was their only desperate way forward.