I don't think the financial played much of a role. I've always thought that the simmering rage that the lust with NK brought to a boil was due to CW's deep-seated, life-long passive and conflict-avoidant nature, which allowed a strong, naturally assertive person like SW to steam roll him completely. Neither of them was aware of this toxic dynamic which was a cancer to their relationship.
The passive man who does not have it in him to assert his own wants and needs becomes dejected, then resentful, then angry, then rageful. Each instance where he doesn't speak up and lets her needs/wants take precedence, he gives up a little bit of himself. Its not healthy.
Where does this originate? First I believe CW was born with a quiet/introvert/passive temperament. There is lots of research on how temperament is what people come into the world with. He gets this from his dad. I would guess there are others on the Watts side that have this same personality style. Introverted/anxious/good with his hands/fixing things/not so good with other people's emotions, or his own. A man like this usually pairs up with an extroverted woman.
Then CW has his mother Cindy who does not have the capacity to connect properly with her son or appreciate his nature and create an environment for him to flourish. CW goes through life being steam-rolled by his mom, while his dad sits by and lets it happen, because he has the same nature as his son, and the first woman that responds to CW's interest, he attaches himself to. SW probably felt very familiar to him, and in the beginning, it was fine to take the back seat and let SW run the show.
Its not SW's fault that she was running the show necessarily, because I believe CW never spoke up about his needs. I think SW thought everything was fine. At the very end, in her texts, you see that she was open to counseling and changing. I think SW would have been willing to change had she known the depth of her husband's feelings of a lack of agency in their relationship. I think CW ended up creating this false life for them where he went along with everything, but the resentment slowly built year after year.
Then he gets on Thrive, loses weight, has a lot of energy, gets in shape/builds muscle, which increases testosterone, and CW is feeling confident and probably has a stronger sex drive than when was overweight, out of shape, and no vitamins and other stuff being pumped through him all day. NK comes along, pursues him, and he spirals out of control.
We all know how a woman can treat a man if she wants to - fawn over him, make him feel like the #1 guy in the world, be interested in everything he likes, etc. I think NK and CW did have some natural common interests, but I also think she was amping it up due to lust. Also, NK feels very familiar, subconsciously, to CW - she is yet another confident woman and his passive nature provides a great complement to that.
CW had literally NO ability to voice his feelings and needs and assert them. It was not possible in his childhood due to both nature and nurture, and he unconsciously chose someone who helped him stay in that role as an adult. By the time he had spent six weeks with NK, in fantasy affair land, where there is only lust and the idea of a perfect future, he was quickly reaching the moment it was all going to fall apart, in a spectacular way - the baby coming, SW is back home, NK has been pressuring him, his desire for NK is at an all-time high, and his deeply passive nature at the root of it all.
I almost believe him when he said there was a force that came over him when he killed SW. I think that force was the deeply buried part of him that had wants and needs, and had reached a point of rage. Strangling SW is symbolic in that he closed her throat and prevented her from speaking or breathing. That is the thing he had wanted to do all those years and never could do. Not because of SW, but because of his own nature. In that moment, the repressed aspects of CW burst out in rage and he shut her up for the final time.
