Kevin is not a sociopath, but rather a psychopath. The author and psychologists have made that clear. His mother did have a part in it, due to her SLIGHT lack of affection. Nobody could understand how hard it is to completely change your life like that. She tried, and anyone should be able to see that, but having a child that suffers from a lack of empathy would make almost anyone's efforts seem pointless. Yes, more could've been done, by both the mother AND the father, such as seeing a psychologist for one, but that didn't happen. It could've possibly helped or still lead to that same ending.
I definitely hate the mother's lack of will for herself though. Doesn't speak up or act at all. It's irritating. People in that town had their right to be upset, but she didn't deserve to be treated that way and I would've slapped people back.