I'm not an expert in mental health-meaning I have no idea why any one person commits a crime. Some people steal for economic reason, some steal because their hungry, some people steal because their taught too, etc. When I was a kid I stole a knife from an Army Surplus store because at the time it seemed like a cool thing to do in the eyes of my "group".
Mothers that kill their newborns-likely post-partum depression-who knows?
Why do people that start huge fires say it's like having an orgasm.
Some people are simply filled with anger-give them a weapon and there's a tragedy.
Parenting? I've thought often as to why couples that should be having children are not and those that should not are.