Well, I can excuse family, no matter how far up the tree.
I don't actually disagree with you. At least not totally. I believe that if caught early enough, at risk kids or kids diagnosed with conduct disorder, but still young, can be treated or criminal conduct prevented through a change in their brain waves. I'm basing that on things I have read by Dr. Robert Hare.
But while you may not be using it as an excuse, per se, I think some kind of do. At least, in the sense that due to whatever diagnoses the perp had, he or she had no choice. Except in extreme cases, I disagree with that.
I also think a lot of people jump to grasp onto a psychiatric or psychological explanation for such evil acts because then they can comfort themselves that none of their kids or loved ones could do such a thing. Or, that if they did, there would be a rational reason. A mental problem of some sort.
I know you said you don't believe in evil (IIRC). The alternative though, is lack of choice because all evil is is extreme, profound immortality. I believe that in 99.9999999999999% of murder cases, choice exists.
That Finnish study on teenagers who murder linked above would suggest the probability that this killer is either personality disordered (sociopathic, narcissistic, borderline, which is not insane, IMO), or not mentally ill at all. Only 7% of the homicidal teenagers they studied were schizophrenic.
I have to defer to experts though, like Pensfan and Rougelatete. Hopefully, they will reach a consensus!!!!!!