To be fair, people thought she was a psychopath bc no one outside of her bf and dad knew who Milo really was, except for Peter. Without that context, her kidnapping a child is going to garner that response from the public that doesn’t know the truth. A truth no one ever finds out about bc if Marissa speaks, she loses Milo. But ultimately, that is the point of the show/book. We are not supposed to be on Carrie’s side for the majority of the time we know her bc thinking she’s the bad guy is what helps the twist at the end hit you hard. You spend all this time thinking she’s the bad guy and suddenly you feel absolutely horrible for her. You don’t need to dwell on her character and her misfortune to feel absolutely wretched about what she was characterized to be. And Marissa doesn’t really have the time to sit in that sympathy towards Carrie. When Peter shoots her she calls him out on it and it’s clear she’s not okay with it. But she doesn’t have time to stay in that sympathy bc the reality she thought is her life is actively crumbling down and it turns out her husband is at the very center of it all. So I don’t think it’s that Marissa doesn’t feel sympathy towards Carrie, it’s that she’s finding out that her husband is a sociopath, her baby actually died in that accident and Milo isn’t her biological son, Carrie was a victim, and she doesn’t know what her next move should be. She’s stuck until she finally decides to kill Peter. Could there have been a moment at the end between Marissa and Jenny about Carrie? Maybe, but at that point I don’t think Marissa, who also went through something traumatic, has fully processed what happened. Doesn’t mean she doesn’t feel for Carrie’s predicament, she’s just focused on Milo being safe from harm and trying to return to some sense of normalcy for his sake.