Darren believes he has always had a strong moral sense. Some of this may be his nature, but in his adult life it comes out of his father's actions. Of course, one doesn't have morality without responsibility and guilt. Indeed, this is a key question. How does one live with a subjective morality without the guilt? He doesn't understand how his friend can commit what he sees as an immoral act with its accompanying guilt. The lines blur for Darren. What is wrong becomes more subjective. His experiment in the end is to see if he can live with the guilt of his actions. If guilt is part of the immoral act. He is held frozen by the prospect of the freedoms provided when guilt is absent. If he can commit the immoral act, the faithless act, without being held captive by his own guilt, then his future opens up. His morality, his sense of responsibility has locked him into his course in life. One should question if he can return, not only to his wife, but to his work as well. His work in law, in the area of contracts in particular, is rooted in his sense of moral justice. With that center gone, his work will lack meaning.
There is also the question of religion in the novel. Its blows within the text are glancing (and more of a concern for Nicole than Darren), but one cannot discuss morality without at least the undiscussed notion of Christianity. That Darren and Nicole are not religious, are lacking faith, opens them up to the moral questioning we witness. Not that faith, by itself, would wipe away their doubts about the righteousness in morality. But maybe some of Nicole's despair is 'the sickness unto death'.
Nihilism, I think, is also at play in the novel. Darren encounters people along the way who reject the standards by which we live, judge. He sees this as narcissism. An excuse to live by one's independent standards in rejection of one's inherent moral responsibility to others. This moral order is also part of his foundation, and in the end this might be more of a determining factor for him than any sort of inherent right or wrong.
I don't know that I can see Nicole's struggles in the same terms. She lost her notion of morality in her youth. Her feelings toward Darren are not guided by morals. She does not wonder whether her actions are right or wrong. Things for Nicole are more personal, less abstract. More existential and metaphysical. She does have trouble in her perception of the world. She seems to know that she sees the world differently, experiences the world differently. She feels its imposition. This part of what separates her from those around her. This difference.
But her real problem is the sort of void she feels within. There is a hole that, in her twenties, she seeks to fill with drugs and sex, experience. Later she chooses marriage and child-rearing to fill the void. And we come across her she is again realizing her dissatisfaction. Her thinking of life alone is not the end. It is not what she's after. Its a sort of displacement. A way of trying to fill the void with fantasy, another life. The internet itself is part of this alternate existence. It has not taken over her real life. Her cleaning is a way to constantly order her life, seeking to control and contain it. As much as possible. But life is an imposition. The outside world, the suburban city blocks, even the houses and lawns of the neighborhood, are part of an externally imposed order. What philosophical school of thought all of this falls under, I'm not sure.
Of course, some of my favorite novels are philosophical novels. I didn't set out to write one, though. And I certainly don't think I'm trying to forward some sort of treatise. The realization, though, has given me another way to look at what is happening within the pages of text.