Was it the mother's fault the boy was so soft?

It looked more like just just left, walked off in2 the darkness.

I’ve read the novel twice and watched the film three or four times and in no way feel the mother was responsible for anything outside of whatever trauma was inherently transferred to the boy due to her sudden departure.
As far as the boy being “soft,” I don’t feel that he was, he was merely a young boy, innocent as well as naive but no more that most young boys are at his age, and that was the point the author was making, how do you protect and nurture innocence in a world that has quite literally become a living hell.