I think you need to consider the feelings mama had for Kevin from the start. She never had that normal attachment mother's feel for children. And we know it isn't just about her because she does have that attachment for her daughter.

I don't recall the name, but there's a known disorder where for reasons unknown occasionally a mother will just not have the normal feelings for their child. These children have enormously increased chances of being a sociopath or other major disorder. Gotta think this is what Shriver was writing about.

And just as in real life, it's difficult to lay the blame. Was she uncaring for her son because he was a little baby sociopath, or was it the lack of caring that made him develop into a sociopath? I don't think Shriver answers this question. Though do note at the end how much Kevin still wants his mother's love.