I will agree with your comments to the extent that it is an ongoing debate and topic of research. And clearly, the outcome of an impaired child despite the best efforts of truly dedicated parents is not only a tragedy, but it underscores the bitterness that would be expected to be directed towards anyone who attempted to correlate an impaired child (that was essentially "born that way") with indifferent parents.
However, that being said, I will stick by my assertion that there is a definitely strong connection between "Impaired parenting" and an "Impaired child".
Case in point..the topic of this thread. While the evidence is still being gathered, there appears to be a great deal of information which is basically labeling the shooter as a "troubled child/adolescent/young adult" and a mother who not only collected firearms (including assault rifles???) but who apparently regularly brought her "troubled son" to the range for target practice.
Impaired parenting? The evidence will be forthcoming, and perhaps at this point I have become too subjective to make further comments.