I have taught or been involved with through my current job, several families where a teenager/young adult has killed/seriously injured someone. Not once has the family background been stable. In every case there has been (choose a mix): neglect, alcohol, drugs, abuse, domestic violence, previous offending by parents (violence/petty crime, dealing), low levels of education, mental health issues, gangs. Some have been from known criminal families, some from apparently normal, ordinary families - but it has been a surface presentation.
One had parents who were well-off, hard-working, professional but what emerged was a father who assaulted and controlled the mother, the mother took it out on the younger children in various ways and the whole household functioned on a knife-edge. The younger boy had already come to social services attention because the school had raised concerns about his violent treatment of other children and his lack of empathy for the hurt they suffered, and the unusual response of the parents' to the school raising those concerns.
One came from a family where the father has spent decades creating himself a reputation in the community as a 'lovable criminal'. He's dangerous and his whole family live (partly) on the proceeds of crime (car theft and associated trade). He's racist, controlling, violent (when he needs to be) but also a man who helps others - eg spends the year buying toys, wrapping them and storing them in a spare room and gives away bags of toys to charities and families in need at Christmas. Loves animals and fundraises for a local animal welfare. Helps old people in need. A big chunk of his community adore him, hang on his every word, turn to him when they need help and his family love him to bits but know what he is. The family just think their lives are normal. He has sons who have served significant prison sentences for serious assaults, his daughters never went to school from about 13, and 1 of his grandsons is currently locked up for a knife crime and one is awaiting trial for assault, his grandaughters (now teenagers) don't go to school
Never come across one where there were not serious issues in parenting.