She's an only child who, like Williams, didn't want children. That does tell us something about her ... my opinion only ... but I think she probably is "all about me". I think when Williams confessed, her first thought was for herself, her finances, her reputation, her future, her comfort. Appleby says they hadn't been intimate in years (police info), so it seems they enjoyed each other's company when they happened to be living in the same town, but otherwise they weren't much more than roommates.
I think there were a lot of options for Williams to deal with his sado-sexual problem. He had suppressed it for 44 years. If he hadn't had such a high profile in the military, he could have sought out S&M clubs and fetish groups to have a more socially acceptable way of dealing with it ... and I only say this because he did have control over it for 44 years. I think his high profile career left him afraid to seek help from anyone or find an outlet anywhere.
I think his wife should have been more on the ball. They had just moved into a new place, and Williams had a bag and a computer printer box that he moved from his old place to the new place ... both filled with women's underwear. When she was unpacking the moving boxes and bags, why didn't she find these things? She was there for a month pretty much on her own ... I find it surprising that she didn't grab all the boxes and bags to unpack. I also suspect he wore her underwear at some time during the marriage ... which should have sent some little red flags up all over the place.