The biggest challenge here is that none of the three main characters - mom, dad or Kevin is likeable in any real way. What the book succeeds at is somehow making the parents seem like worse humans than their son, who is a mass murderer. To do so she's had to make the parents into the most clueless entitled assholes possible. The theme seems to be "If you don't like kids, it's probably a good idea not to have them."
There are a lot of people on the Internet saying the book is about nature vs. nurture. But Kevin is such a victim of child neglect that I'm simply left to wonder if taking him to a doctor at some point would have solved most of the problems going on. Here are some events that happen that I think would lead most people to take their child to the fucking doctor. These parents don't do so in any of these situations.
1. When every single nanny hired for him when he's 2 quits in under a week.
2. When he's still in diapers at roughly age 7.
3. When his mom finds herself screaming at him when he's 2 that he makes her want to kill herself.
4. When he throws bricks off of highway overpasses toward oncoming cars.
5. When he starts whipping his dick out in front of is mom and masturbating while looking at her.
6. When he only engages his mother in conversation when she's talking about school shootings.
7. When he comments that if you want to be a school shooter you should do it before you are old enough to be tried as an adult.
8. When he blinds his sister with drain cleaner.
9. When, after talking tons about school shootings, his only interest becomes in getting better and better at hitting targets at long range with projectiles.
10. He falsely accuses a female teacher of molesting him.
Of course a parent smart enough to do that would probably also stop using words like "fags" and "faggots" when their son is clearly gay or bisexual. (They find him just about to butt fuck his best friend when he's about 12, he shows no interest in girls, and wears super tight clothes that are designed to show off his cock. Very cliche, but also clearly what the author is trying to telegraph.)
Perhaps the biggest flaw in the book is that it goes to the old trope that "it's all mommy's fault." Kevin is clearly mostly a kid yearning for attention. He quickly learns that to get attention from dad he has to pretend to be interested in thing's dad likes. To get mom's attention, however, he has to be an asshole, because that's the only time she pays attention to him at all. That he ends up being a school shooter should be no surprise since she talks about school shooters with awe about him all the time. Of course, she's insane. When her husband tells her that having a second child would be a bad idea since she clearly hates children, she then tricks him into getting her pregnant and is shocked when he's not thrilled. But I wish the book got across the idea that the dad is at fault too. He could have taken the kid to the doctor. He could have divorced the "ice queen" he married. He could have realized that having a child with a woman who just isn't into it was a really bad idea in the first place. So, no, this is not just mommy's fault.
The real fault is a society that works to convince people who are disinterested in having children that "it's different when they are your own." Some people simply shouldn't have kids.