no expression of remorse

I was very moved when Kitty said to Walter "I never pretended to be more than what I was." I think it was in the way that she said it. In my mind that exchange with Walter was sincere and heartfelt, probably more intimate than any apology coming from her and I think Walter appreciated that most of all. This is when I began to root for them as a couple.

Eh, she did show remorse. It was subtle.

The first time she visited orphanage, she learned what Walter actually did there. I think she started to think that there is one side of Walter she never knew, so she re-evaluated everything. At the night, she attempted to talk to Walter. If she never regretted anything, she would just stay silent, never talked to each other until Walter died.

Unless you referred specifically about her affair with Charlie, then yes, I agree. But they've discussed about it actually.

Kitty said that it's not solely her fault, because as what Walter said, he even knew that Kitty didn't love him when they got married. He knew the risk but he still married Kitty anyway.

Also during scene when Walter put the package near Kitty' bedroom, Walter didn't confront Kitty and Charlie, but putting up indifferent facade instead.

So they two are actually responsible of their marriage life. Kitty is that kind of person and Walter this kind of person. They failed to see what can make them together and too focused on differences between them.

Kitty thought Walter is too serious. Walter himself never tried to be more sociable towards Kitty (since he's the quiet and serious kind) so Kitty felt that Walter didn't care much about her. That's why Kitty was attracted to Charlie.

Walter' job field is also something that Kitty isn't familiar with. That's why when she visited orphanage, she just realized how much Walter tried helping people--even though Walter sees himself "studying bacteria" instead of helping people.

Consider the scene in which Kitty informs Walter that she is pregnant. Kitty is weeping and literally says "I'm sorry" when she admits that she is not sure who the father is.

You don't necessarily feel sorry about an affair. It was what you wanted, but you can feel sorry that you hurt someone. Even then toy might not stop. The pull off the other person is too great.