I don't think that's what the movie suggested at all. Kevin's mother had no idea/understanding of how to deal with him, so her nurturing became fucked up, which helped lead him down the path he went. I mean, she broke his arm when he was around 4 or 5 years old, they were clearly showing that nurture was a part of it.As a small child, Kevin wasn't evil, just seemed to lean heavily in one direction on the autism scale.
I think what the film makers wanted, is for the audience to debate just how much influence his mother had on him. If she could have done something different, that would have changed things. Something to that effect.