Interesting thread. The ambiguity of the book being whether the mother's failure to bond caused Kevin to become the person he did and act so terribly, or whether he was "born evil" so to speak, to the extent that his own mother reviled him, is an extreme example of the nature/nuture debate.
We've all heard of children born into terrible backgrounds of violence and abuse who use this as a "how not to" example and pull themselves out of it, becoming successful parents and citizens. Equally there are children born into relative wealth, privilege and stability who will go off the rails, testing their parents to the limits with drug use, theft, abusive behaviour etc.
Surely however it's far more likely that those with poor life chances will go on to have a poorer life than those born with more advantages who will benefit from their stability and education. So nuture, in my opinion, has a much bigger role to play than nature.
Clearly we don't yet know the scope of genetics - is there a dominant gene that predisposes you to criminal behaviour, and this gene will out no matter what social conditioning you've been put through? Is there a gene which would cause a child born into a decent, if not overly loving, family with enough money for good quality housing, food, education, leisure etc to inevitably act as Kevin does?
Equally it seems that the early years are being given increasing importance in their role. It used to be thought that the first 5 years were absolutely vital in shaping a child's future, more recently it has been put at 2 years being the key. So if a child is neglected, rejected, abused before he or she is 2, then nuture is effectively shaping nature.
What I think is problematic is that the Kevin story is all about his mother. What about the father? He is weak, vacuous, reluctant to probe into the depths of his son's and wife's minds and motivations. He sees a happy family because he wants to, but doesn't seem to do much to promote one. He puts his head in the sand about Kevin's early behaviours rather than tackle them as an involved and concerned parent would. Parenting seems to be left to the mother alone - true of many households today? Possibly.
The mother obviously fails to bond (and the subtext screams PND at us) to the extent that she feels as though Kevin is a complete stranger. What effect would that have on a child - to feel always rejected, pushed out? No wonder he acts as he does - to an extent. Most people who suffer rejection on this scale would be likely to have some problems in life, but how many set out to kill and maim as Kevin does?! Ultimately that choice is his own and can't be laid at his mother's door.
Finally, it's while since I read the book, so I can't remember if there is any mention made of the mother's own parents. If Kevin's actions can be blamed in any part on his mother, then her inability to love her son (though she bonds with her daughter), may well relate back to her own childhood. Nuture again, rather than nature.