The differences between a film and a novel can often be very illustrating as to the nature of the seemingly common material they mine.
<i>Yes but I would have preferred the film to have delved even more into the effect rather than the cause</i> - some dude
The effect is Eva in the state we see her in for the entire duration of the film. The movie isn't about a woman trying to cope with an impossible situation so much as a woman who feels she is being punished, over and over again and has stopped trying to fight it. The difference in Eva pre and post killing is only a matter of degree. She was in hell the second Kevin was born and, with that specific act, he made sure she would remain there for the rest of her life.
Too much of the film consisted of Kevin's cartoonish lowlights (played mostly for shock value or black humor) for my tastes but the more I think about it the more I realize that everything we see is from Eva's perspective. She always saw Kevin as something to be endured, not loved, and so interpreted everything he did as a personal affront to her. The Eva in the novel is able to hang on to the occasional glimpses she catches of his humanity (when he does not realize he is being observed, when he is sick, during his television interview, during their final encounter in prison) and realizes how exhausting it must be for him to maintain such a complete front of apathy against the world and of contempt for her. The Eva of the film, either unwilling or unable to at this point in time (and who can blame her given the final fuck you that is Kevin's choice of weaponry), endures her punishment as if she deserves it.